Sheaves of G-structures and generic G-models

In this article we give an equivariant version for the construction of generic models on presheaves of structures. We deal with first order structures endowed with a suitable action of some fixed group, say $G$; we call them $G$-structures. We show that every exact presheaf of $G$-structures $\mathcal{M}$ has a generic (equivariant) $G$-model $\mathcal{M}^{^{gen}}$.