There may be other specifics buried in Part 95 rules, but here is one that pertains to GMRS repeaters and certain base stations. The way I understand it is the transmitter used in a GMRS repeater must be FCC type accepted as a Part 95 repeater and here is one specification called out for GMRS repeaters that is different than mobiles or hand helds:
(b) Each GMRS transmitter for mobile station, small base station and control
station operation must be maintained within a frequency tolerance of 0.0005%.
Each GMRS transmitter for base station (except small base), mobile relay
station or fixed station operation must be maintained within a frequency
tolerance of 0.00025%.
That would mean part of the FCC type acceptance for a GMRS repeater transmitter would include meeting 0.00025% frequency tolerance. That usually requires a high end TCXO or ovenized oslillator, something that most mobiles and hand helds usually do not have.
prcguy