I don't have any experience with compiling Java so I'm just leaving this here for future me.

1) Download Java JDK (Any version should be fine)
2) If you're lucky to have a .gradle file in your app directory, download Gradle. (You can then add gradle to your PATH, but that's optional)
3) Go in the directory where your .gradle file is situated and run

PATH_TO_YOUR_GRADLE/bin/gradle build

4) The compiled .jar will be situated in build/libs

That's it!