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.
0 - https://about.gitlab.com/pricing/
I honestly couldn't even figure out how to buy an on-prem license.