Hi, former gerbil owner here. I’d say most gerbils can’t pass for the average mouse on a good day, but if you don’t really know what you’re looking at it’s easy to be confused. Plus some country mice do have slightly varied coloration that makes it not un-possible to confuse the two species.

Honestly though, gerbils don’t usually give a fuck. Mice are usually pretty skittish. I can totally imagine a domesticated gerbil coming up to a human and going “Hey.”