As a native English speaker from England I can say I've never heard or seen this work before! I haven't seen Gravity Falls, either, but after a quick Google search I found that the word 'grunkle' is actually made up, and the creators of the show have put the words 'great' and 'uncle' together to make 'grunkle.' Hope this helps! :)

It is a mix of Great and Uncle, not Grandfather and Uncle as someone else said. Stan is the main character's great uncle, (the brother of their grandfather)

i think GRUNKLE means grandfather and unkle mixed together