Both tools carry the Gemini name, run on the same models, and now sync with each other. They are still built for opposite situations, and picking wrong is the most common way to waste time with either one.

The question to ask is simple: does this task have sources, and does the answer need to be traceable?

A Rule of Thumb

Reach for a notebook whenever the phrase "according to what exactly" would be a fair response to the answer. Contracts, papers, policies, course material, competitor documentation, meeting recordings: anything where the source matters as much as the summary.

Reach for the app when the sources are in your head, on the open web, or nowhere yet.

Where the Two Meet

Ten files is the ceiling for a Gem in the Gemini app, and a Gem's files are reference material rather than a citation-backed set. When your material outgrows that, or when every answer needs to point at a passage, the work belongs in a notebook. Since notebooks now appear inside the Gemini app, moving between the two costs nothing.

One Habit Worth Keeping

Grounding removes a category of error, not all of them. A notebook will not invent a source, but it can still misread one. The citation exists so that checking is cheap, and the entire value of the tool collapses the moment you stop clicking them.