Dipper saw McGucket fall and that's enough to get him running over.

"Grunkle McGucket what's...." He looks over and sees. Oh wow, that sure is a familiar coloured beam of light "Oh, no."

Dipper has seen the portal back home. He's seen it work and bring Ford back, but he's also read the journals, the full journals and how McGucket got pulled into it years ago. So he turns to McGucket to try to...help? He's not sure, just, he's family now and that's what matters.

"Can you hear me? Grunkle McGucket! It's okay, it's not going to pull you in!"