Hi Laura! Thanks for bringing your question to planDisney.
While I have not brought a GoPro handler grip myself to the park, I have friends who have. So, I'm happy to pass along that you should be able to bring a GoPro with you. You are correct to note that selfie sticks and hand-held extension poles are
prohibited, however, and would not be allowed through security. If you use a non-extending handle to stabilize your hold, you should be okay to enter with the accessory. However, security and Cast Members on Attractions can always use their discretion about permitting its use based on safety concerns. You may be asked additional questions about your accessory or you might be asked to put it away on some Attractions. Just keep that in mind and be sure to be courteous with filming: Some Attractions prohibit video lighting for the enjoyment of other Guests, which would be disrupted by obtrusive lighting.
When you want to be in the picture, be sure to take advantage of the talented
Disney PhotoPass photographers, who are ready to capture your magical moment in locations throughout the parks. Sleeping Beauty Castle at the end of Main Street, U.S.A. in Disneyland Park is an iconic location, but my favorite spot is in Star Wars: Galaxy's Edge. We've used PhotoPass photos from our trips for more than just a couple of holiday cards! They're that good.
Have a magical time.
Elizabeth