The advantage of using Gradle as the build system for our LeJOS projects is that we can use any text editor to write the programs. This does not mean that you are bound to do so. Using an Integrated Development Environment (IDE) has many advantages, some of which are stated as follows:
Syntax highlighting (highlights syntax errors)
Code completion (offers possible options while you type)
Code inspection (you can easily navigate to class definitions)
A project-level view of the code
These advantages are worth the effort required to set up the IDE. Any IDE that supports Java can be set up to work with LeJOS. If you are familiar with an IDE, all you need to know is that you need to specify the locations of the source code. One location is obviously the root folder of your project. In addition, you need to direct the IDE towards the ev3classes and DBusJava folders that contain LeJOS's ev3 libraries. The build.gradle file does the exact same thing.
I recommend that you use an IDE only to edit the source code. Leave the compilation to Gradle and the automation of the compile-transfer-execute cycle to the Make utility.
Let me discuss a few popular Java IDEs (others are configured similarly). In case you are not inclined to read through the whole list, my recommendation is Android Studio (detailed at the end).