Principal GL(n)-bundles **are** equivalent to locally free sheaves of rank n, but not in the way you're describing. The easiest way to see this is to note that principal G-bundles have fibers that look like G (in the case of GL(n), these would be n²-dimensional), but locally free sheaves of rank n have n-dimensional fibers.

The way you get a locally free sheaf of rank n from a GL(n)-torsor P is by twisting the trivial rank n bundle O<sup>n</sup> (which has a natural GL(n)-action) by the torsor. Explicitly, the locally free sheaf is F=O<sup>n</sup>x<sup>GL(n)</sup>P, whose (scheme-theoretic) points are (v,p), where v is a point of the trivial bundle and p is a point of P, subject to the relation that (v&sdot;g,p)&sim;(v,g&sdot;p). It happens that the map is bijective. Given a locally free sheaf F of rank n, the automorphism sheaf Aut(F) is a GL(n) torsor (and this procedure is inverse to the one I described).

Similarly, if you have a group G **and a representation** V, then you can associate to any G-torsor a locally free sheaf of rank dim(V). I don't know of a characterization of which locally free sheaves of rank dim(V) arise in this way.