As far as I can tell, the regs say that a fuel gage is required and
operable. It doesn't say that it has to be accurate.

My entire primary training (some years ago) was done in ancient 150's,
152's, and 172's whose fuel gages were notoriously unreliable. Unless
the regs (or their interpretation) have changed since then, it's
probably okay to fly with an inaccurate gage.

You may want to check with the FBO/mechanic/owner to find out what they
really mean by "inop." If it's not reading the correct value, you're
probably ok. If something is broken, then the plane's probably
unairworthy.