I seem to recall that GCJ had a lot of limitations back in the day - you couldn't use the same standard library, it didn't support newer language features, etc. Wouldn't surprise me if even now GCJ had poor integrations with IDEs and other essential tools to make Java livable.

GCJ was abandoned in 2009 as the majority of its devs left to either work on Eclipse's compiler or early OpenJDK days.

It is still available on gcc, because of its unit tests. Some gcc code paths are only used by gcj.