We do know that they ported "graphd", the Freebase database, to Google's infrastructure. Really nothing happened at Freebase for a year and a half while they were doing this.
They've probably created a very large Freebase graph for scale testing and it's likely that they're able to handle 500M entities from a hardware perspective.
If you look at how often the GKG comes up in search, it clearly doesn't come up very often. So the process that makes the visible GKG is a process of subtraction, not addition. There are many topics where the GKG could give you an answer from Freebase but it doesn't, and they prune many of them away because they don't want to take the chance of giving wild answers. It's more likely that 1 or 1.5 million topics are in the visible GKG.