two circuits on a single pole GFCI breaker?
If it's on one breaker, it's one circuit.  

It's true that you often see circuits wired as daisy chains, one linear thing, like a vine. Actually, we wire in "tree" topology, a vine is just one kind of tree.  You are welcome to have as many branches as you please.  What's more, there's nothing wrong with those branches splitting right at the service panel.  

Can you land two wires on one breaker? Maybe. It all depends on whether the breaker is listed (UL tested) for two wires, and this must be in the instructions or labeling.  For instance, Pushmatic breakers are indeed listed for 2 wires, it's one of their charms I will miss.  If the breaker is not listed for two wires, then you have a 6" pigtail to a wire-nut. 

Yes, you are allowed to wire-nut inside a service panel.