Yes, that second video is pretty much exactly what I'm looking for. I've been considering implementing it myself as a general-purpose git tool (i.e. not github etc. dependant) - were there any major gotchas? Do you need to pre-cache commit or diff info. or is it efficient enough to do everything in real-time?

The major gotchas is building the indexing engine. Once you have that in place, getting the commit information takes milliseconds. The only time consuming thing is extracting the from and to blobs from the repo. However, that is a one time hit, since you can cache the blobs.

For the diff, this is done in realtime with https://github.com/Microsoft/monaco-editor

Also my product isn't really dependent on GitLab or GitHub. It works in standalone mode as well.