There is no certification for amateur radio equipment -- we operators are responsible for the technical details of our transceivers. So yes, it's perfectly legal for us to modify equipment designed for other services and use it on the Amateur Service. We can build our own radios from scratch, or using anything as a starting point, including a radio designed and sold for another service. The problem comes from the other direction. On GMRS, you must use a certificated transmitter, according to 95.129. A GMRS-Certificated transceiver must meet the requirements of Part 95, including 95.621, listing the frequencies permissible for GMRS operation. Part 95.655 specifically calls out that if the radio can transmit in the Amateur bands, it can't be certificated for GMRS. § 95.655 Frequency capability. (a) No transmitter will be certificated for use in the CB service if it is equipped with a frequency capability not listed in § 95.625, and no transmitter will be certificated for use in the GMRS if it is equipped with a frequency capability not listed in §95.621, unless such transmitter is also certificated for use in another radio service for which the frequency is authorized and for which certification is also required. (Transmitters with frequency capability for the Amateur Radio Services and Military Affiliate Radio System will not be certificated.) It's legal for us to to modify a GMRS-certified radio to use it on the Amateur Service, but if that's done, then it no longer meets the certification requirements for use on GMRS.