There's plenty of GM sfz soundfont on the net just find one you like.
You can play the midi with the soundfont vst easily if you have all midi tracks on the one track but if you want to have separate tracks use this script, it will send the same channel that each track is on to the same channel on the vst soundfont track.
Select all the midi tracks you want to send to the GM Soundfont track, run the script and select the GM Sondfont track.
ReaTrak send selected midi tracks channels to track x.lua
You can use VSTSynthfont with a different soundfont on any channel
http://www.synthfont.com/Downloads.html (donation ware)
or free BassMidi
http://falcosoft.hu/softwares.html#bassmidi_vsti
https://www.softpedia.com/get/Multim...idi-VSTi.shtml
and load a GM sfz