You cannot program the GM300 directly with a computer. You need a dedicated programming cable.
Yes, Radio Doctor was released as freeware by independent hobbyists within the radio community. It is not an official Motorola product. Because it is legacy hobbyist software, it is no longer actively updated, but archive copies are shared freely across radio forums and enthusiast repositories. Hardware Required for Programming motorola radius gm300 radio doctor free
Even with Windows-based software like Radio Doctor, a slower, older PC with a physical, native DB9 Serial (COM) port is highly recommended. USB-to-Serial adapters often fail because they cannot replicate the precise voltage timing required by the radio's EEPROM. You cannot program the GM300 directly with a computer
You do not pay $150/hour for a bench tech. You become the doctor. it is no longer actively updated