It is not clear that this is the app that will be used for the new watches. I imagine it will support the new RePebble watches, but I believe that app was intended for the original Pebble watches.
The thing that makes it so unclear to me is that this is a repo owned by the Rebble team, not the RePebble team. I do not know how much overlap there is between the two teams, but the RePebble team does not have any open source repos that I could find. Any mention of open source software by RePebble (including the OS) are links to repos owned by other teams, which is a little concerning.
I believe Google plans to use Google Play Services to block side loaded apps. By default, GrapheneOS does not come with Google Play Services installed. I am not sure how things would work if the sandboxes version of Google Play Services that GrapheneOS provides is installed.
The issue about maintaining/updating GrapheneOS is a separate issue from side loading apps. That was due to Google shifting the development of Android to a closed source model and only open sourcing the final code. This limits the Grapheme team’s ability to anticipate changes and make any required adjustments until after the release of Android.