Github wikis and GitLab wikis are both just Git repositories containing text files, so you can just pull from one and push to the other.
Go to any page on your Github wiki and click the Clone URL button. You'll get a URL like https://github.com/Homebrew/homebrew.wiki.git
. Clone it to your computer:
git clone https://github.com/Homebrew/homebrew.wiki.git
cd homebrew.wiki
Then, on your GitLab wiki, click the Git Access tab, find the URL in the instructions (in the first line under the Clone Your Wiki heading), and push to that URL:
git push https://gitlab.com/adambrenecki/test-project.wiki.git
If you can't find the URLs, they should be roughly the same as on this page, with the usernames/repo names changed as appropriate.