Does Canvas detect Gemini?
No
Canvas cannot see what you had open in another tab. It sees the file you submitted.
How it works
Browser activity outside the page is not visible to a web application. Only a locked browser or an installed proctoring agent changes that.
The part people miss
If an integrated text detector is enabled, Gemini-drafted prose is scored like any other model's output.
Flagged for something you wrote yourself?
There is a right order to handle it in, and editing the document first destroys your best evidence.