Is it legal? NO. Has anyone ever been prosecuted? Probably not. My own thought is use of a non certified radio on GMRS is probably not a primary offense. That is, the FCC is extremely unlikely to come after you for this reason alone. In fact, if the radio is operating properly there’s is no way anyone could tell it was not certified just by monitoring it. However, if the FCC had other reasons to go after you, like spurious transmissions, too much power, being a nuisance or other observable violations. Then, they would probably add a charge for use of a non-certified radio to whatever else they were pursuing. That is 100% my personal view and does not reflect this site or, possibly, even my view tomorrow.