  Motivated by the problems of interpretation of a nonlinear evolution equation
in quantum mechanics we discuss in this contribution the concept of nonlinear
gauge transformations, that has recently been introduced in joint work with
Doebner and Goldin, in the framework of Mielnik's Generalized Quantum
Mechanics. Using these gauge transformations we construct linear quantum
systems in a ``nonlinear disguise'', and a gauge generalization of these (in
analogy to the minimal coupling of orthodox quantum mechanics) leads to a
unification of Bialynicki-Birula--Mycielski and Doebner--Goldin evolution
equations for the quantum system. The notion of nonlinear observables
introduced by L\"ucke is finally discussed in the same framework.
