You are aware that GitLab is MIT licensed and self-hosting is a strongly supported use case, right? Maybe I misunderstood the "selling" verb there
I honestly couldn't even figure out how to buy an on-prem license.
If you go to the pricing page[0] and click the “Buy GitLab <tier>” you’re interested in purchasing (Premium or Ultimate), a pop up will appear and you can purchase an on-prem license by clicking the “Purchase self-managed” button.