Can I put gypsum and paint finish on top of a concrete fast set repair mortar?
I would use a setting joint compound (maybe durabond) rather than mud because it's more durable and moisture resistant.  It's more time-consuming to sand, however.  Either should stick to concrete and both should get a coat of primer before painting.  If have old or really glossy concrete, you can prep the surface with a little Muriatic acid to etch the surface and get a better bond, but that shouldn't be necessary for fresh-cured mortar.