Goku is slightly stronger than Dipper (74 tons vs 68 tons), but he doesn't have any control over the Oozaru form whatsoever, making it a lot easier for Dipper to avoid his attacks
Dipper was also able to tank hits from Rumble, who stomps him in AP, and even survive his ultimate combo finisher without lasting injuries (though it did ko him). Oozaru Goku also doesn't have the mouth blast like Nappa or Vegeta, so this is also out of the question
Besides, Dipper can shrink Goku down with his flashlight, something he did before and experienced himself by having Gideon do it to him. The AP gap between Goku and Dipper isn't really big enough to allow Goku to overcome his clear disadvantages in intelligence and skill (since Oozaru Goku doesn't use martial arts so it's not relevant here)
Dipper should win this handily, but not easily