Every professional has a writing task they repeat weekly. The sales email they send every Monday. The client report they rebuild from scratch every Friday. The LinkedIn post they procrastinate on every Tuesday morning. A custom GPT trained on their voice eliminates that task permanently — and someone needs to build it. That someone can be you.

Full overview: AI Side Hustles — The Complete Guide →

What Is a Custom GPT Side Hustle

A custom GPT is a version of ChatGPT configured for a specific use case — trained on a specific writing style, given specific instructions, and built to produce a specific type of output. OpenAI’s GPT Builder lets anyone create one without code.

You find professionals with repetitive writing tasks, build them a custom GPT, and charge a setup fee plus a monthly retainer for updates. The initial build takes a weekend. Maintenance takes 1–2 hours per month.

Who Buys Custom GPTs

Real estate agents: Property description generator, client follow-up emails, open house scripts
Marketing consultants: Client report writer, campaign brief generator, social media caption tool
Executive coaches: Session summary generator, action plan writer, LinkedIn thought leadership posts
Lawyers: Contract summary tool, client intake emails, brief outline generator
E-commerce owners: Product description writer, ad copy generator, customer email responder

Step 1 — Find a Client

The fastest path: LinkedIn. Search for the profession you want to target. Send a simple DM:

“Hi [Name], I build custom AI writing tools for [profession]. I can create a GPT trained on your voice that handles [specific task] in seconds instead of 30 minutes. Happy to show you a demo — takes 15 minutes. Worth a quick call?”

Send 20 messages. Expect 2–3 responses. Book a demo call. Show them a GPT you built for a fictional version of their role.

Step 2 — Build the GPT

Go to chat.openai.com → Explore GPTs → Create
In the “Instructions” field, describe exactly what the GPT should do, the tone it should use, and the format of the output
Upload examples of the client’s existing writing (emails, reports, posts) as knowledge files
Set the conversation starter prompts to the specific tasks they need
Test it with real inputs until the output requires minimal editing

A good custom GPT setup takes 3–5 hours for a new client. After your 3rd build, you’ll do it in 90 minutes.

Step 3 — Price and Package

Setup fee: $300–$800 depending on complexity
Monthly retainer: $75–$200 for updates, refinements, and support
Bundle: Offer 3-month prepay at a discount to lock in recurring revenue

With 5 clients on a $150/month retainer, that’s $750/month in near-passive income after the initial setup.

Step 4 — Deliver and Retain

After setup, schedule a 30-minute onboarding call. Walk the client through the GPT. Create a 1-page PDF showing how to use it. Check in at day 14 and day 30 to gather feedback and make adjustments.

Clients who see measurable time savings become long-term retainer clients and referral sources.

Combine With AI Chatbots

Custom GPT services pair naturally with customer-facing chatbot services. Once you understand a client’s workflow, you can offer both. Full guide: How to Build an AI Chatbot for Your Business →

Requires a ChatGPT Plus subscription ($20/month) to create and share custom GPTs. OpenAI’s features and pricing may change — verify current terms before pitching clients.