I do not keep up with the details of particle physics.

Gell-Mann is responsible together with Pierre Ramond and Richard Slansky and independently of Peter Minkowski Rabindra Mohapatra Goran Senjanovic Sheldon Lee Glashow and Tsutomu Yanagida for the see-saw theory of neutrino masses that produces masses at the large scale in any theory with a right-handed neutrino. He is also known to have played a large role in keeping string theory alive through the 1970s and early 1980s supporting that line of research at a time when it was unpopular.