Redgard is a fluid applied waterproofing and crack proofing membrane that is typically used to seal shower floors and walls where you have installed either cement board or tile backer board. It can also be applied to brick/CMU/plywood. I have never tried to use it on greenboard and don't know if it is warrantied for that application, or how paint would then bond in a ceiling application.

In a nutshell it is a roll-on plastic liner you can put mortar over and then tile. I have also used it on a plywood subfloor, under a LVT in a laundry room to guarantee any leaks/spills/overflows would not get to the joists and subfloor.

Pre-Catalyzed epoxy is a one-part pre-mixed epoxy paint. Most waterproof epoxies are 2-part kits that you have to mix together then apply (messy and a PIA to deal with). to form a hard waterproof coating. Pre-Cat comes premixed and ready to use like standard paints or shellacs, but it forms a harder wear surface and is more durable at sealing, especially on GWB/greenboard. Plus, it can be tinted to color match anything. Even the big box store like Home Depot carry it and mix it. If it is an oil-based system (there are water based as well) and it is difficult to paint over down the road. I guess that is a pro and con. it's hard-to-get other paint to stick to it. It does an amazing job of waterproofing drywall/GWB/greenboard.

https://www.homedepot.com/p/BEHR-PRO-1-gal-White-HP-Pre-Catalyzed-Waterborne-Epoxy-Semi-Gloss-Interior-Exterior-Paint-HP15001/301899214