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 (By similarity). ERG is one of the major redox buffers which protects bacteria against redox stressors and antibiotics; loss of ERG or mycothiol (MSH, the other major redox buffer in this bacteria) leads to respiratory alterations and bioenergetic deficiencies that negatively impact virulence (PubMed:26774486). gamma-L-glutamyl-hercynylcysteine S-oxide + H2O = L-glutamate + S-(hercyn-2-yl)-L-cysteine S-oxide Amino-acid biosynthesis; ergothioneine biosynthesis. Expressed in log phase, part of the egtA-egtB-egtC-egtD operon, which does not include egtE (PubMed:26774486). Truncated N-terminus.