In this paper, we develop the deformation theory controlled by pre-Lie algebras; the main tool is a new integration theory for pre-Lie algebras. The main field of application lies in homotopy algebra structures over a Koszul operad; in this case, we provide a homotopical description of the associated Deligne groupoid. This permits us to give a conceptual proof, with complete formulae, of the Homotopy Transfer Theorem by means of gauge action. We provide a clear explanation of this latter ubiquitous result: there are two gauge elements whose action on the original structure restrict its inputs and respectively its output to the homotopy equivalent space. This implies that a homotopy algebra structure transfers uniformly to a trivial structure on its The classical computations hold here, so one has ln(e λ ) = λ and e ln(1+λ) = 1 + λ for any λ ∈ Hom(A ¡ , End(V )), which concludes the proof.In this case, the exponential group G exists in the bigger algebra a A,V , and is isomorphic to the gauge group Γ. By the weight grading property, its action on Maurer-Cartan elements is well defined and given by the conjugationSince the Maurer-Cartan elements correspond to homotopy A-module structures, the question is now how to give a homotopical interpretation to the gauge group and its action. Let α and β be two homotopy A-module structures on V , i.e. two Maurer-Cartan elements. One defines a more general notion of maps between two homotopy A-modules, called ∞-morphisms and denoted α β, by degree 0 maps f : A ¡ → End(V ) satisfying the equationThese can be composed by the formula g ⋆ f and ∞-isomorphisms are the ∞-morphisms such that the first component is invertible, i.e. f (0) (I) ∈ GL(V ). When this first component is the identity map, we call them ∞-isotopies and we denote their set by ∞−iso.Theorem 1. For any Koszul algebra A and for any chain complex (V, d), the group of ∞-isotopies is isomorphic to the gauge groupand the Deligne groupoid is isomorphic to the groupoid whose objects are homotopy A-modules and whose morphisms are ∞-isotopiesProof. The first assertion follows directly from Proposition 2. Two Maurer-Cartan elements α and β are gauge equivalent if and only if there is an ∞-isotopy between the two homotopy A-module structures on V . Indeed, there exists λ ∈ Hom(A ¡ , End(V )) 0 such that β = e λ .α = e λ ⋆ α ⋆ e −λ if and only if