Can I safely remove gouache from an acrylic painting without damaging the acrylics?
Yes. Acrylic dries impervious to water, and gouache remains
water-soluble, so you should be able to dissolve the gouache with
water without affecting the acrylic. However, if the gouache was
applied before the acrylic was completely dry, there may have been
some mixing or staining that is not reversible. Some pigments tend
to stain the surface more than others (phthalo blues and greens for
example). Test on a small area.