In almost of its $d=4$ vacua, string theory contains Yang-Mills degrees of freedom or light spin-one particles so that in the form of the string field theory, it may be rewritten as a field theory with a gauge group, a generalized Yang-Mills theory with infinitely many fields charged under the gauge group. The gauge symmetry is exact in this formulation because it's what removes the unphysical polarizations of the gauge fields.
However, string theory is not a local quantum field theory and the gauge symmetry isn't a fundamental assumption in string theory – it and the corresponding polarizations of the gauge bosons are derived from something more fundamental, from the maths of string theory (e.g. from conditions of the world sheet conformal field theory if we deal with the string theory perturbatively). General relativity also follows from string theory (much like Yang-Mills theory, the diffeomorphism invariance is exact whenever we rewrite string theory in a form that includes $g_{\mu\nu}$ as the degrees of freedom) but it's a derived result, an infinitesimal glimpse of the superior power of string theory.
It's hard to answer your question because the phrases you use are ill-defined – in particular, you don't say whether a gauge theory has to be a local quantum field theory in spacetime (string theory is not one). But I believe that the paragraphs above answer all meaningful questions that one could get by clarifying yours.
