  We present a phenomenological analysis of available data on single spin
asymmetries for pion and $D$ meson production in proton-proton collisions at
RHIC within the color gauge invariant generalized parton model (CGI-GPM), which
includes initial (ISIs) and final (FSIs) state interactions in a transverse
momentum dependent formalism. This study allows us, for the first time, to put
preliminary constraints on the two independent gluon Sivers functions entering
the model. We also present a comparison with the simpler generalized parton
model (without ISIs and FSIs).
