GS means the bank also contains additional GS instruments. GM means the bank contains only the 128 standard instruments, and a standard drumset. The size is based on the total size of the uncompressed sf2 file, and may vary slightly. Anyway, as far as I know, here are all commercial GM soundfont banks that have been released. My YouTube channel has some recordings of MIDI music (mostly DOOM) played with these GM banks. Please note that it is impossible to fully explain the quality of a GM bank in text, and you may or may not agree with my opinions here. So let's take a look at the commercial GM soundfonts that have been released. Besides these custom GM soundfonts, there are also commercial professional GM soundfonts ones that are generally meant to be very high quality, and of course cost money.

They mostly use free or public domain samples, which are taken from various sources. Squillions and squillions.that's about how many free custom General MIDI soundfont banks have been released.

Roland sound canvas sound font