The Motorola GM300 is great commercial radio and can often be found used for a very reasonable price, but it's not the easiest radio to interface to amateur gear.
This adapter plugs into the GM300's 16-pin accessory port and adapts it to the 6-pin Mini-DIN packet data connector common in the amateur radio world. Many TNCs can be connected directly to this port, and our trackers can use it with the CT-M1 cable.
Amplification is provided for the transmit audio, and a trimpot allows adjustment of the receive audio level. The SPK jumper can be removed to mute the radio's internal speaker.
The GM300's receive audio is provided on both the '9600' and '1200' pins of the Mini-DIN, which is wired as follows:
1 - Transmit audio
2 - Ground
3 - PTT
4/5 - RX audio
6 - Squelch
WA8LMF's excellent packet connector documentation
This adapter should work with other Motorola radios having the same 16-pin connector, but be sure to check your pinouts first and make sure you correctly identify pin 1 before connecting the adapter.