Gell-Mann was apparently very picky about how people pronounced his name. So you'd better do it right:
It has a hard G. It does not sound like jello or jelly. Second, and almost as important, the two syllables are pronounced with nearly the same emphasis. The accent is not put on one syllable or the other, it is put on both. It is probably easiest for us Americans to deal with if we just think of it as two words. That's about all there is to it. (A lot of people just called him Murray.)