This is exciting. I think code forges are one of the biggest opportunities for ActivityPub to really go mainstream and change the internet. Not only because it'll make working with open source way easier since you can work with any compatible forge, but developers will be more exposed to ActivityPub just by working with the software and so more likely to participate in AP dev. It will be interesting to see what effect this has on the fediverse. There's been a lot of talk from various organizations/companies but this will be the first large project adopting AP. I'm interested to see how development goes for them and for other fediverse projects.
I wonder what changes it will force on Mastodon. Masto won't be the biggest project anymore and won't be able to throw its weight around as much. Just like the recent influx of users forced the implementation of full text search and has reenergized conversations about quote posts, I think federated gitlab would force masto to rethink some things.