We explicitly compute the
differential Galois groups of some families of generalized confluent hypergeometric
equations by a method based on the asymptotic analysis of their irregular singularity
at infinity. We obtain the Galois group directly from a particular set of topological
generators. These are formal and analytic invariants of the equation, reflecting the
asymptotic behaviour of the solutions. Our calculations yield classical groups as well
as as the exceptional group G2.