Drone ( the only standalone non-SaaS CI system I've used) was easy to install and connect ( it was a matter of 3-4 config options, and a single auth/app creation flow).

It's not that it's hard to setup external CI for something like gittea, it's that GitLab CI works REALLY well and is deeply integrated. I have no doubt that gittea/drone can work, I just doubt it can work as flawlessly as gitlab and be maintained and operated so seamlessly (seriously i've never seen a software package as complex as gitlab be as simple to install/maintain/upgrade over years).