I'm going to agree with them. I had one as a kid and while I didn't ever see it catch a gnat I can pretty much guess how it would go.
They'll snap shut at the lightest touch so you get one maybe two gnats in there and it shuts. Then, at least with mine IIRC, they always took 30 minutes to an hour to reopen even if they didn't catch anything.
I also don'tthink they'd keep up but, depending on how many gnats you're seeing, and how many VFTs they might. They are cool as hell though so honestly it ain't gonna hurt to try.