unfortunatly the pinetree image you see in the poster is not meant to be a reference to Gravity falls.
It's based on an upcoming game, Firewatch.

too bad we probably won't get a full-on crossover, seeing how half of the shows are owned by Disney, and half by Cartoon Network
not to mention the CN ones are divided into the regular network ones, and the Adult Swim ones, meaning different ratings and target demographics

still, the little hints continue being great