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)                                   ||

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