How can I use my utility classes in different git-projects at once?

Question

I'm a beginning programmer and I apologize if my questions is trivial but I haven't found a sufficient answer to my problem.

I have a git repository called "toolbox" with some utility classes that I frequently use in other projects. Until now, I have manually copied those class files from this project in other projects whenever I needed them.

This if of course not a good way of doing it. I frequently add new features and fixes in whatever project I'm currently working on to these files. It makes version management a nightmare.

What I'd like to to is to import the toolbox-classes directly into the IntelliJ-Project(s) (which is also on the same git as the toolbox repo) and whenever I make a change to those files in the toolbox-repo I want the other projects to be able to automatically pull those changes as well.

If possible I'd also like to be able to share my toolbox-repo easily with others who might need those classes. But that is not a requirement. I'd just be nice to be able to do so.

I tried to use git submodule. It included the entire toolbox-repo in the target-repo but unfortunately I wasn't able to use the utility classes. I asked someone more experienced and they told me that I need to define "SourceSet" in the gradle.build but I wasn't able to configure that due to my lack of knowledge.

How can I include/import/use my utility classes from my toolbox-repo within other projects?

Thanks for any advice.