Agency New Business Proposal GPT: Train AI on Your Best Pitch Work

What This Builds

A Custom GPT trained on your agency's best work: credentials, case studies, voice, capabilities, and past winning proposals. When an RFP arrives, you feed it the prospect's requirements and get a structured, agency-voiced first draft in minutes instead of days. The difference between responding to 3 RFPs per quarter and 8.

Prerequisites

• A ChatGPT Plus account ($20/month). Custom GPTs require Plus.
• 3-5 past winning proposals or RFP responses (remove any confidential client data before uploading)
• Your agency's credentials deck or capabilities document
• 3-5 case study write-ups (can be simplified versions of what's on your website)
• Your agency's "About Us" and positioning language

The Concept

A Custom GPT is like a new coworker who has read every strong proposal your agency has ever written and knows your voice by heart. When you give it a new RFP, it doesn't start from scratch; it starts from your agency's best thinking, drawing on real case studies and established language. You set it up once and every proposal starts 80% of the way there, needing editing rather than writing.

Build It Step by Step

Part 1: Prepare your source documents

Before opening the GPT builder, prepare your source material:

1. Agency capabilities summary: 1-2 pages max: who you are, what you do, what makes you different, your team structure. Keep it factual and current.

2. Case studies (3-5): One page each, in this format:

   • Client: [Industry, company size, anonymize if needed]
   • Challenge: [What problem they brought us]
   • Our Approach: [What we did]
   • Results: [Quantified outcomes]
   • Relevant to: [Types of prospects this is most useful for]

3. Voice guide: 1 page: 5 brand voice attributes for your agency itself (how you write proposals), with examples of on-brand and off-brand language. ("We're confident but not arrogant. We say 'We've driven results for X' not 'We are the best agency in the region'")

4. RFP response samples (2-3): past winning proposal sections. Redact any proprietary client info, but keep the structure and tone.

Save all these as separate text or Word files; you'll upload them to the GPT.

Part 2: Open the Custom GPT builder

1. Go to chat.openai.com → sign in to your Plus account
2. Click your profile icon (bottom left) → "My GPTs"
3. Click "Create a GPT" (top right)
4. Select the "Configure" tab, which gives you full manual control

Part 3: Write the GPT's core instructions

In the Instructions field, write:

You are a new business proposal writer for [Agency Name], a [describe your agency: "full-service independent advertising agency specializing in B2B and healthcare brands" or similar].

YOUR JOB: When given an RFP document or brief, write structured agency proposal sections that sound like us — not like generic agency boilerplate.

HOW TO RESPOND TO AN RFP:
1. Read the RFP carefully. Note the required sections and any specific questions asked.
2. Write each required section using our voice guide and drawing on relevant case studies from our knowledge base.
3. Flag (in [brackets]) any places where I need to add specific information you don't have.
4. At the end, list the 3 case studies from our knowledge base that are most relevant to this prospect.

AGENCY VOICE: [Paste your voice guide here — 5 attributes with descriptions and examples]

CRITICAL RULES:
- Never claim capabilities we don't have
- Always root strategic claims in specific, real results from our case studies
- Don't use overused agency phrases: "best-in-class", "holistic approach", "full-funnel solutions", "thought leaders"
- Write in first person plural ("We believe", "Our team", "We've done this for")
- Proposals are direct and specific — not padded with vague aspirations

Part 4: Upload your agency documents

1. Scroll to the "Knowledge" section in the Configure tab
2. Click "Upload files" and upload all documents you prepared in Part 1:

   • Agency capabilities summary
   • All case studies (one file per case study, or combined into one document)
   • Voice guide
   • Sample winning proposal sections
3. Wait for each file to process (shown by a green checkmark)

What you should see: Your uploaded documents listed with green checkmarks in the Knowledge section.

Troubleshooting: If a file is too large, compress or summarize it. PDF files work; Word files work. Keep total upload under 20MB.

Part 5: Test with a real or mock RFP

1. Save the GPT (name it: "[Agency Name] Proposal Writer") → save as "Only me" for privacy
2. Open the GPT from your My GPTs list
3. Upload a recent RFP document or paste a summarized RFP brief
4. Prompt: "Here is an RFP from a regional hospital system looking for brand advertising support. Write the following required sections: Agency Overview, Our Approach, Relevant Experience, Team Overview. Note any gaps with [brackets]."

What you should see: Proposal sections written in your agency's voice, citing specific case studies from your uploaded documents, with bracketed notes where you need to add specifics.

Part 6: Refine and tune

After your first test:

• Review every section: Does it sound like your agency? Is the voice right?
• Check case study citations: Are the right case studies being cited for this type of prospect?
• Note where it defaults to generic language. Add more specific voice examples to your instructions.
• Run it on 2-3 more RFPs before declaring it production-ready

Real Example: Healthcare RFP Response

Setup: Agency capabilities doc, 4 case studies (2 healthcare, 1 B2B, 1 retail), voice guide, 2 past winning proposals, all uploaded.

Input: "Here's an RFP from a regional healthcare system in the Southeast, looking for a brand agency to help them position their new cancer center. They want to see: Agency credentials, our approach to healthcare brand strategy, relevant healthcare experience, and proposed team. Write these sections."

Output (excerpt from Agency Credentials section): "[Agency Name] is a [city]-based full-service advertising agency with 12 years of experience in healthcare and professional services marketing. We've partnered with regional health systems, specialty practices, and pharmaceutical brands to build the kind of trust-based brand presence that drives patient acquisition and physician referrals.

Our healthcare work delivered measurable results: for [anonymized regional medical center], our campaign repositioning their cardiology program drove a 34% increase in consultation requests within 6 months. [Continue drawing on uploaded case studies...]"

Time saved: From 4-12 hours of RFP writing to 1-3 hours of editing and customization.

What to Do When It Breaks

• Case studies aren't being cited correctly → add more specific labels to each case study file: "HEALTHCARE CASE STUDY: [brief description]" at the top
• Voice sounds too generic → strengthen your voice guide with more "sounds like / does not sound like" examples; specificity is everything
• Sections are too long → add to instructions: "Keep each section to 200-250 words unless the RFP specifies otherwise"
• Missing required RFP sections → prompt explicitly: "The RFP also requires a section on Diversity, Equity & Inclusion. Write that section now."

Variations

• Simpler version: Don't build a Custom GPT. Instead, keep a "proposal prompt document" in Google Docs with your agency credentials, case studies, and voice guide. Paste it into Claude with each RFP. Same principle, slightly more friction.
• Extended version: Add your agency's pricing model and rate card (internal only) to the GPT knowledge base so it can draft Investment sections with real numbers. Only do this on a private GPT you fully control.

What to Do Next

• This week: Upload your documents, test on 2-3 past RFPs (ones you know the outcome of), and calibrate the quality
• This month: Use on your next real RFP and track time savings and win rate compared to fully manual writing
• Advanced: Build a second GPT variant for a specific vertical (healthcare, financial services, retail) with deeper case studies and specialized language for that category

Advanced guide for advertising agency account manager professionals. Requires ChatGPT Plus subscription at $20/month.