The Knowledge Graph is growing quite quickly as we add more and more data sources. Freebase is one of those sources but there are obviously others that make up the full 500M entities.
The rate at which we add entities to the Knowledge Graph is different from the rate at which those entities appear in search. What you see in search now is just part of the Knowledge Graph but we're continuously increasing the coverage while making sure that the quality of the data remains extremely high. I guess that's what you would call the "visible GKG"; to us, its all one big graph.