I'd be hugely surprised if it was anything with GCN limiting VPNs. That's complex and not in their interest. As a broadcaster, they want more viewers. Even the original rights owner only cares a little. The loser is the broadcaster with rights for the area you're proxying out of, and they're not really in a position to act on it. Yay for internet anarchy. In this case.
Not all VPNs are created equal. Perhaps there were some tech issues, traffic limits, or other connectivity issues with your VPN service provider that day.
I watched GCN yesterday to see Wout decimate the field on the beach at Middelkerke. They must have had US broadcast rights because no VPN was required. But it worked when I turned VPN on for a bit, too. I assume I'll need to use it in a few weeks to watch some of Fayetteville from "Manchester."