I think G'Kar took a few episodes to come into his own; essentially, once Straczynski stopped using him simply as a heavy and allowed for more wisdom and introspection ("Mind War," I think?) he became much more interesting. The same thing might be said about Crais on Farscape, of course. He's effective in "Premiere," but one-dimensional. He gets some important backstory in "That Old Black Magic" and only grows from there.
The thing with G'Kar was intentional though. It was pre-planned that he'd have his character arc where he'd start off as a two dimensional villain and you'd get to watch for five years as he slowly becomes <spoiler redacted>
. Indeed, most of the main cast had similar arcs, though with the exception of Londo, none were quite so drastic.
And yes, the 'Mind War' b-story (specifically the "ant" speech) was the first glimpse we're given into what's really going on with him.
Crais on Farscape was a totally different situation. It was a dead-end, one dimensional character that they kept around largely because they liked the actor. His motivations appeared to change with the wind and you can really tell that they had no clear plan. Don't get me wrong, he was a good character and he became very interesting towards the end, but I wouldn't compare him to the likes of G'Kar on anything other than superficial terms.