add gzip easyconfig for gmpolf toolchain, clean up comments in gmpolf toolchain

@Bart-CER: I added an easyconfig for `gzip` being built with the new `gmpolf` toolchain, as a (simple) test to make sure that the toolchain support is working.

I tested a build of `gzip` with the `gmpolf` toolchain on our end, and it's working as expected, so once this PR is merged in, we can merge in your 3 PRs so they'll be including in EasyBuild v1.9.0!