I think its simply a reselling of GCP storage services with overhead cost of their backup redundancy and SRE on-call.
But even with all that, 6GB per year definitely make you question the value of using Gitlab Container Registry vs Your Cloud of choice similar offering.
They simply know the storage limit is the one users will most likely exceed first so having all of it bundled instead of separate price for storage and for "compute" would net them less money.
From their perspective it's basically "as long as it is cheaper than customer paying someone to set up gitlab instance"
But even with all that, 6GB per year definitely make you question the value of using Gitlab Container Registry vs Your Cloud of choice similar offering.