The GM300 was Motorola's next step in the MaxTrac / Radius mobile product lines. The schematics are remarkably similar. You can even interchange some boards between the GM300 and MaxTrac radios. Like the MaxTrac, the GM300 line has been discontinued by the manufacturer. Throughout this article, reference to MaxTrac radios implies Radius radios as well.
Naturally, you need different programming software (RSS), but if you've ever programmed a MaxTrac, you'll be right at home with the GM300. The radios operate the same, too. See below for more info on the RSS and programming.
GM300 mobile radios cover the VHF (136-174 MHz in two ranges) and UHF (403-520 MHz in four ranges) bands, with 8 or 16 channels, 12.5 or 20/25/30 kHz channel spacing, and 10, 25, and 35-45 watt power levels. They use the same accessories (loudspeakers, microphones, accessory plugs, power cords, mounting brackets, etc.) as the MaxTracs.
The M120 radio is just about the same as a GM300 but has "less features" - this would be the 2-channel version, equivalent to a MaxTrac 50. There is also an M10 single-channel radio, and an M130 2-channel radio that is an M120 with the GM300's signaling capabilities.
The 16ch GM300 radio uses the expanded logic board with firmware in an EPROM, that gives you the ability to program several of the accessory jack pins. The 8ch GM300 and the M-series all use the masked logic board that has no programmable pins. The masked logic board also has limited code plug space, hence fewer channels and less capabilities.