A Custom GPT is a version of ChatGPT with instructions, uploaded reference files, and sometimes external tools already baked in, saved so you (or others, if published) can reuse it without re-explaining the setup every time. Think of it as a saved configuration rather than a separate product.
Building one is mostly writing a good system prompt: who this assistant is, what it should and shouldn't do, and how it should format answers. Attach reference documents — a style guide, a product spec, an FAQ — and the GPT pulls from them automatically instead of relying purely on general training data.
Actions take it further by connecting a GPT to an external API, so it can look up live data or trigger a real action instead of just talking about it. That's the difference between a GPT that describes your return policy and one that can actually check an order status through your backend.
The clearest use case is a repeatable task you or your team does often enough that re-explaining context every session wastes real time — drafting support replies in your brand voice, summarizing a specific type of report, or answering common internal questions from a known set of documents.
Skip building one if the task is a one-off; a well-crafted single prompt is faster to write than a dedicated GPT you'll only use once. And if you publish a GPT publicly, remember any files you attach are part of what it can reference — don't upload anything you wouldn't want a stranger's prompt to potentially surface.