Gitlab is, AFAIK, nowhere near the scale of Github. Shopify IDK, but I'm fairly certain their type of usage is very different. Basecamp is also of another scale, and certainly a very different usage/performance type.

Shopify primarily uses MySQL, unless something major has changed recently. They've done a number of conference talks and engineering blog posts about their usage of MySQL, see e.g. https://www.google.com/search?q=mysql+site%3Ashopify.enginee...