What version of rules_go are you using?

v0.27.0

What version of gazelle are you using?

v0.23.0

What version of Bazel are you using?

4.0.0

Does this issue reproduce with the latest releases of all the above?

Yes

What operating system and processor architecture are you using?

darwin/amd64

Any other potentially useful information about your toolchain?

Nothing special, using Command Line Tools rather than Xcode.

What did you do?

Use $(GENDIR) in cppopts.

What did you expect to see?

The "Make variable" get expanded to bazel-out/darwin-fastbuild/bin.

What did you see instead?

$(GENDIR) is provided verbatim to the Go compiler, without any expansion.