TagBar seems to check the version of ctags before it runs the language specific ctags (go tags), here. As a result, it will complain, that ctags is not "exuberant ctags". If you install ctags, via brew install ctags, then TagBar will proceed to work, and call gotags.

Perhaps this should be filed on tagbar rather than here, however simply noting the issue in the readme.md might be helpful for others. The error message seemed to imply the problem was with gotags, and not ctags itself.