OK, if you manage to mess up the logic board and for some reason decide you want to make the GM300 into a Maxtrac, you can follow this procedure. It does require modifying the circuitry in the radio to get it to work again.
I, personally, would rather you check out the GM300 page and look at trying to re-initialize the radio as a GM300, rather than going to this drastic step of making it a Maxtrac (and losing some features on the way).
The first thing you need is a firmware EPROM for a 32 channel Maxtrac. Install it in the GM300 in place of the GM300 firmware.
Following that, you need to blank the logic board with Maxtrac Lab RSS. Be sure to choose "Extended Codeplug, Clear Tuning Data". Then, go into the Replace Logic Board menu, and re-initialize the board as a Maxtrac High Signalling board, with 32 channels, and the proper bandsplit and power level of the original radio. Be sure to select the panel number as #001, and remember to enter the serial number of your radio correctly. Complete the initialization by selscting Program (DON'T program with old tuning data).
Your radio should now be a Maxtrac and use Maxtrac RSS for programming. You will be able to add all you modes in and program as required. You will note that the radio has no audio, keep reading.
In order to get the audio back, you will need to open up the radio and do a modification. Pull both covers off the radio. Copy the crystal data off the sticker in the RF section so that you can do the proper, full, RF alignment when you are all done (because right now your radio is set to all defaults).
On the logic board side of the radio, locate the IC indicated in the diagram below. This should be a 9 legged audio IC.
CAREFULLY cut pin 8 or desolder it from the PC board. Leave enough leg so you can solder to it. Solder a 100k ohm resistor between your now free pin 8 and pin 7.
This will restore your audio. The only problem is that the audio will be inverted. You will not notice the difference during normal conversation, but, if you program the channel with receive DPL, you have to remember to program them with Inverted DPL (receive) instead.
Re-assemble the radio and you are all set.
Just go through the tuning procedure in the manual and you have a GM300/Maxtrac hybrid that is all ready to go.