S80 can play SMF (type 0) midi files using internal playback-only sequencer reading them from a RAM smartcard or from midi. You cannot save,edit sequences as S80 doesn't have sequencer builtin.
GM is a standard arrangement of voice position so standard midi files can use GM banks that have the same voices in the same positions as in midi files. In a non GM keyboard midi (GM) files have from minor to major differences (for example midi file have a part in grand piano but sound module has in the same position some other voice).
GM is useful to you if you want play ready made GM midi files.
In S80 you can use the PLG100-XG expansion board that adds a 32 voices 16 part GM/XG synth in the main keyboard. These extra 32 voices are 32 plus S80 (64) voices so the polyphony (in theory) becomes to 96.
I must say to you that Yamaha GM/XG voices are made from maximum two elements so the sound of this board is not as good as S80's excellent banks. Anyway GM/XG plugin is useful only for computer made midi files playback.
You must not take non GM as an S80 limitation. The keyboard is excellent. I have Rolands (JV2080,XP80) a new Triton Pro and many other sampler-module racks but this new keyboard is very good. I has excellent sounds (especialy if you need natural piano/pad/organ sounds), easy editing (for a Yamaha and a two line LCD), easy live setup, and the best 88 keys weighted action keyboard. And at a friendly price
If you have any other questions please post them here.