Quick take
Gamma is useful when the output should be a deck, visual explainer, proposal, one-page site, or client-ready draft. It is strongest when you bring a real outline or source material. It is weakest when you ask it to invent strategy, numbers, or positioning from a vague prompt.
Best fit
Use Gamma for sales decks, founder updates, course explainers, internal docs, lightweight sites, and client proposals that need structure and design faster than a blank PowerPoint file.