I bought a Decocolor gold paint pen/marker, but is says it is oil based, will that be safe on my PYO’s gypsum?
Yes, in small areas it works fine. We use Deco paint pens for the small gold and color spots on the dragons.
Oil based, or lacquer paint won’t hurt the gypsum, but if the piece is entirely sealed with the paint, it is possible it may blister.
Use plenty of ventilation! Those pens have really nasty ingredients in them.