Trying to directly translate a Java-style workflow to .Net is probably going to be tough sledding; it's just not how things are done. You can do it, but you'll fight everything more than being on the happy path. The dotnet CLI tooling is relatively slick, from what I've seen.
Basically everything. In my web projects, gradle runs css and js minification and compilation, and in game projects, it runs the asset preprocessing.
The build-time code is handled with gradle’s buildSrc folder.
I also obviously use Annotation Preprocessors to pre-generate code for my Java projects, and additional build tools to further minify and optimize the output (e.g., for Android, I also run redex over it).
Basically, gradle for me handles the entire build – fetching dependencies, building native libraries, running code generators, building Java code, binding them together, processing assets, compressing the result, and optimizing it all – and deploying the results.
