I wouldn't use Gallagher as a measure of where stand up was then vs. now.
Chappelle and Chris Rock are probably the only guys still performing I would put up there with Rodney Dangerfield, Eddie Murphy, Richard Pryor, George Carlin, Redd Foxx, Don Rickles...
Comedy first got really weird from the mid-70s to early 80s. Steve Martin and Andy Kaufman were playing Carnegie Hall. Cheech & Chong had 3 best-selling albums. Albert Brooks was making Johnny Carson spit up his coffee. And you had Gallagher, who at that time, was wildly different, and initially, pretty damned entertaining.
Gallagher's problem, some of which he mentioned in the Maron podcast, was that his audience demanded the watermelons, and he buckled to that instead of evolving, thus he became a niche act as comedy moved away from jokes to the storytellers of the mid-90s through now.
But make no mistake, Gallagher had a solid three years or so from '78-'80 where he was a must-watch act.