Trying to integrate Patchouli into my mod; added the repository and dependencies to my build.gradle file. However, when I run the game, and try to load a world, loading ends up stuck on 100%. The game doesn't crash though, so I have no crashlog to provide. However, the error reported is here. I've changed nothing about Patchouli's source, and I was testing on a stable version of my own mod. All I've done is added the maven repository and dependencies to my build.gradle file.

Versions: Patchouli 1.16.2-47 / Forge 35.0.1
MC 1.16.4

have you read https://github.com/Vazkii/Patchouli#mixin-troubleshooting?

I have. I may be doing it wrong (and I apologize if I am, I'm somewhat new to modding in general), but I placed the line property 'mixin.env.disableRefMap', 'true' inside my build.gradle file, but it didn't do anything, the same issue persisted. I don't know where else to put this, or how else it should be written. Again, I apologize if this is me making a noob mistake.

Well, my own project currently depends on Patchouli and I have no issues with disabling refmaps. The actual line I use to disable them differs a bit from yours though:
https://github.com/Extegral/Enigmatic-Legacy/blob/1c56e9374b0737b510e92fd9b1b63eee7b4b5f2c/build.gradle#L54
Not sure if that matters but you can try changing it to that.

That didn't do anything either, I still got the same error. I don't know what I'm missing, if anything at all.

I dunno, are you sure you're not doing something dumb? (like starting client instance from within Eclipse rather than command line)

Well I'm using IntelliJ, but yeah I am starting it from within the IDE using configurations. That's how I've been doing it and I haven't run into problems doing it that way until now.

Edit: tried running from command line and it worked... I didn't realize I was doing it wrong since I never had a problem running the game through the IDE.

Using IDE runs is not an issue on itself, but I suspect you just never updated your runs after generating them. Use gradlew genIntellijRuns (or whatever is it, not sure I remeber exactly) to re-generate your IDE runs anytime you do any tweaks to runs in your build.gradle. Ideally you should do this even when you just update Forge/MCP or do any major changes in general.

^