Catalyzes the hydrolysis of the gamma-glutamyl amide bond of hercynyl-gamma-L-glutamyl-L-cysteine sulfoxide to produce hercynylcysteine sulfoxide, a step in the biosynthesis pathway of ergothioneine. Ergothioneine is an antioxidant that protects mycobacteria from oxidative stress. gamma-L-glutamyl-hercynylcysteine S-oxide + H2O = L-glutamate + S-(hercyn-2-yl)-L-cysteine S-oxide Amino-acid biosynthesis; ergothioneine biosynthesis.