Can I paint on top of the slick surface of gloss medium?

Yes. The majority of acrylic gels and mediums are pure binder, and acrylic paint will stick to them regardless of the sheen. If you are unsure of adhesion, you can lightly abrade the surface, but if you're working with good quality acrylic paint it shouldn't be necessary. All acrylic paints, mediums and gels are completely compatible.