The 'G' meant that it was built by General Motors Diesel (GMD), the 'F' meant that it was a freight unit, which had gearing differences compared to passenger service, the '30' meant that it was a 3,000 hp. unit. If it was for passenger use, it would have had a 'P' instead, or 'S' for switcher, 'Y' for yard use, etc.
If the first letter had been an 'E', it would mean that it was build by General Electric. General Electric sold off it's locomotive division to Wabtec in 2019.
GMD also no longer exists, having sold that division off to Progress Rail (a Caterpillar subsidiary) in 2010.