never saw them live (!), but with several hundred shows on disc, i can offer the following opinion without bias of attendance:
best years for JG playing: 77, 78, 71, 72. after that, 69, 70 and maybe 80. (FWIW, i LOVE my kezar 75 show, which IIRC is the only show they did in 75).
after that, spotty at best, moments of magic within months of noodling in a song he may not be in key for. the 90s were a sad joke compared to what the dead were in the 70s.
but when he was on? there's a reason jimi hendrix, when asked "what's it like to be the best guitarist on the planet?" answered "i dont know, you should ask garcia"
thing that awes me: just listen to the slide guitar playing he contributed to CSNY's "used to be a king". even when he was just a backing studio player, he could contribute beauty and magic, within 4 rigid bars in a song he didn't own.