Can you paint gloss over matte?

Always prime first and make sure to key (sand) all surfaces, whether you are priming or not that is VERY important.

The rule is: You cannot use water base paints over oil base paint. You may if you prime first. If your matte and your gloss are both water based, you may use the matte. When in doubt, please prime first.

Answer

Being in the painting business for nearly 10 years, the key factor to painting waterbase paint (in any sheen) over oil base paint is to sand, sand, and sand again. A regular primer will not add any adhesion to the surface - you need a high adhesion primer that is either oil or shellac base first, or a waterbase that is made specifically to grip to oil. Of course, I would skip this step entirely simply by sanding thoroughly - and using a waterbase topcoat that is good quality. Ideally you should use a paint that is made to stick to oil, such as General Paint's HP2000 or Dulux Diamond. But priming with a regular waterbase primer will not help to solve this problem and will be a waste of your time and money.