1) Add it yourself
2) Pay someome who is an expert with Gitlab CE code, maybe even certified as expert but doesn't work for GitLab CE.
3) Buy a plugin made by someone on a marketplace, which has reviews, maybe passed a security audit, etc.
In short, I am not saying no one has to charge anything. I'm saying that in an open ecosystem there would be plenty of options. You don't thank wordpress they finally let you add analytics or privacy to your wordpress blog, like you would with blogger.com back in the day. You may still have to pay for the plugin BUT you don't have to wait for an overworked Automattic to build it. You have an entire ecosystem.
And to take it one step further, with open standards you can then have multiple platforms out there to choose from, which can all work with each other.
Look, at my job we self-host Gitlab CE. It's 100% Free Software and we could implement all the features we want without asking them a damn thing. Yet I still have asked them to "please add this feature".
It has nothing to do with being Free Software or not, but with asking them to do actual work in implementing the feature.