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 +-----*            
        +---+