The Motorola GM300 is programmed using the Motorola RSS Software for DOS, which can be sourced online. A ready to use version is available on the Tx86DOS disk image from EI9IUB

The radios are usually programmed through the front RJ45 connector, using a so-called RIB (Radio Interface Box). Most cables purchased online are known as RIB-less, and are essentially USB - TTL devices such as the FT232x with a diode inline between TX / RX.

See below for a Homebrew Motorola RIBless cable schematic:

         FT232 -> Motorola GM300/GM340 (RJ-45)                                   
                                                              
        FT232                              RJ45              
        +---+  1N4148                   1 +-----+            
        |TXD|----|<---+       +-------- 2 |     |            
        +---+         |       |         3 |     |---+        
        +---+         |       |         4 |         |        
        |RXD|---------+-------+   +---- 5 |         |        
        +---+                     |     6 |     *---*        
        +---+                     |     7 |     |            
        |GND|---------------------+     8 +-----*            
        +---+                                                

The rear 16 pin accessory connector:

                                          
                   RADIO TOP                          
       +---+---+---+---+---+---+---+---+              
       | 2 | 4 | 6 | 8 | 10| 12| 14| 16|              
       +---+---+---+---+---+---+---+---+              
       | 1 | 3 | 5 | 7 | 9 | 11| 13| 15|              
       +---+---+---+---+---+---+---+---+              
                  RADIO BOTTOM                        
                AS SEEN FROM BACK                     
                                          
    1 EXT SPKR             9 EMERGENCY ALARM          
    2 MIC AUDIO IN        10 IGNITION CONTROL         
--> 3 PTT                 11 RX AUDIO OUT    <---         
    4 EXTERNAL ALARM      12 PROG I/O 12              
--> 5 FLAT TX AUDIO       13 SW A+ SENSE              
    6 PROG I/P 6          14 PROG I/O 14              
--> 7 GROUND              15 INT SPKR +               
    8 COS (PROG)          16 EXT SPKR +