At the moment bitrise.io only have support for push events(Gogs). This is, I think, because of Gogs popularity.
Because of some internal constraints, we(and maybe others) have to use Gogs and we really need such support for pull requests events. Without this support it’s hard to have a great CI process, isn’t it? 
Definitely a great idea, thank you @viplord for the #feature-request!
Personally I’d really love to see this implemented, as I’m a huge fan of Gogs/Gitea, but indeed Gogs is less popular on bitrise.io so it might take some time to get to work on it. Votes definitely help us to prioritize this higher 
P.S.: as I mentioned in our previous discussion part of the feature can be added to our open source webhook server, to start proper builds / to handle the related webhook event, the only thing can’t be added there is to send back the build’s status. In any case if someone would have some time you can add the webhook support to our open source webhook server at: https://github.com/bitrise-io/bitrise-webhooks
We’re of course happy to help if you’d have any questions 