http://e26whn2524322mkxb3cbyk27ev2ihhq2biz35hty7gzgsyrwrygq27yd.onion/posts/documentation/linux/745-allowing-your-internal-search-engine-to-index-gitlab-issues-commits-and-wiki-pages.html
However, it's indexability is basically non existent - even if you have projects marked as public, significant parts of pages are fetched by javascript, so a crawler can't index things like issue references (no, I'm not kidding) unless they execute javascript. It's like they reverse search engine optimised... It's not just on-prem Gitlab either, it also affects their cloud offering. Search within Gitlab is excellent, but this is of little use if you have things spread...