Give your gloss painted walls a good rub over with a "really rough sandpaper". Spend a bit of time on it,, you need to roughen/dull down the shiny surface. When you have prepared the surface,, get a good quality PVA, (i use builder centre's own brand), and a 25kg bag of British Gypsum bonding coat plaster. Make up some bonding coat,, not too thick, and not too much at a time, say a litre of water and add bonding plaster to it. When you're ready, brush the plaster on, mixed with neat pva on the brush, then let it dry,, 24hrs, then you can plaster over it without any problems. The walls in the pics were painted with vinyl silk, but i have plastered onto oil based gloss and eggshell many times, using the same method described in the post.
Roughcaster.