How "decompilable" a GMS2 game is depends entirely on how the developer built it:
For years, GameMaker Studio (GMS) has been the entry point for indie developers, spawning hits like Undertale , Hyper Light Drifter , and Pizza Tower . With such popularity comes an inevitable side effect: curiosity. Whether it’s a developer who lost their source code or a modder wanting to peek under the hood of their favorite game, the demand for a GMS2 decompiler is constant. gamemaker studio 2 decompiler
Keep in mind that decompilers may require technical expertise to use and may not always produce usable results. Additionally, using decompilers may violate GameMaker Studio 2's terms of service. How "decompilable" a GMS2 game is depends entirely