synthfont64 is a 'player' for want of a better word. you load it as a synth and then add the soundfonts to it. i'm not sure how you are playing soundfonts at the moment so i can't comment on the quality you are hearing - does vlc play soundfonts?. i know it plays videos.

yes, if you compare them side by side, GM soundfonts probably don't sound as good as individually sampled instruments loaded into kontakt or similar. but you can pay a lot of money for top of the range midi sounds

but don't dismiss GM synths. the midi demos in PG Music programs are played on a TTS1 GM synth. have a listen

https://www.pgmusic.com/addons.styles.php?os=win

and you can get the TTS1 free - see Peter Gannon's 'no brainer' sticky post.

the midis on the PG site sound pretty good for a free GM synth don't they? but on pc speakers most things sound a little dud - try headphones