Custom git diff tool in gitlab

Gitlab 12.1 CE. Exist whether way to add custom git diff tool in gitlab? Im look intoGIT_EXTERNAL_DIFF=before call my script on bare repos ( on my gitlab server ). When im test GIT_EXTERNAL_DIFF with custom diff tool - it works on local machine.

So, can, in principle, Gitlab work with custom diff tool?

So the answer is Yes, but it cannot be completed before this ticket is made https://github.com/libgit2/libgit2/issues/1328 after that, add a custom diff tool looks easy.