> is it possible '''not to see Gerrit at all''' and use Github for contributing
Not in general. The mirrors under the wikimedia GitHub account are one way. If you want more info on that, you can ask on wikitech or look on mw.org
. I know it has been asked on wikitech before, so perhaps there are some docs on it now.
> use Github for contributing to semantic extensions
If someone with push rights on the main repo maintains a mirror on GitHub where you can submit pull requests for a particular extension, then that could work. It does however fragment the review queue and makes your code less visible to other developers.
> ''not to see Gerrit at all"
> What is your main motivation for desiring this? (There are a pile of plausible ones ;p)