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.