I'd be much more willing to put Gell-Mann on that list than Hawking or Feynman. To me, his discovery of the ability to make accurate predictions about the existence of fundamental particles based on symmetry groups forms a pillar of theoretical particle physics and the composition of the Standard Model, in the same kind of way that Einstein's application of differential geometry to the concept of space-time does.

Newton will sort of always stand apart in that he was "the first", but I think that grouping Gell-Mann (no pun intended) with Hawking and Feynman, who although both brilliant and huge personalities, weren't the same level of "giant upon whose shoulders others stand" that Einstein and Gell-Mann's work ended up being is somewhat unfair.