> DigitalOcean App platform[…] only connect to GitHub

They also support deployments from GitLab (so long as you're using the gitlab.com-hosted instance and not a self-hosted GitLab instance). If you've deployed your own self-hosted forge, then you can connect DigitalOcean App Platform to it by using gitlab.com as a bridge—register an account on gitlab.com once and instruct your self-hosted forge to replicate copies to gitlab.com. You don't really need to actually use GitLab.

Having said that, considering that DigitalOcean is in the business of selling IaaS/PaaS, it's loony that they don't let you connect to, say, your own self-hosted Forgejo running on their infrastructure…

(Indeed, considering how many people would like to self-host their own forge but how few people want to actually set up and do admin for it, it's loony that DigitalOcean doesn't pick up, say, Forgejo and/or an alternative and offer a sharply discounted (e.g. $20/year) quasi-managed one-click deployment option with first-class support for connecting to their App Platform.)