If a Motorola VHF CB transceiver doesn't turn on, is it dead or can it be repaired?

when you turn a radio on, it is in the "receive" mode, which draws very little current, sometimes less than 1 amp. your DC supply works because it powered the one radio, so the issue with the GM300s is internal, possibly a fuse, but perhaps more serious. The GM300 is not a CB radio and requires government issued permission to operate. It also requires RSS software to program the frequency ignments for the 8 channels. the channels currently programmed in may be illegal for your cl of license. You need professional help in making these radios functional.