So I figured out the problem.
I was dragging off my GameState variable and expecting to see “Cast to IGameClock” but instead I only saw “Cast to BP_GameState”.
Turns out you need to right click the event graph canvas and search for “Cast to IGameClock” and when it’s added to the canvas you can then plug in the GameState.
A future improvement to UE4 BPs could be to show a list of "Cast to " based off the concrete class and the interfaces it implements.