You have a couple of problems.
First, the GoPro footage is in fact 1280x1080. It has a wide pixel aspect ratio, 1.5, as you see in AE.
What is the resolution of the AE comp in which you placed the footage? Did you create the comp by dragging the clip onto the Make Comp Icon, or a different way? This is the most likely cause of your problems.
Second, you have a frame rate mismatch between Premiere and AE. The footage is 23.976 frames/sec, but the premiere project is at 29.97 frames/sec. As long as you don't change any frame rates in AE, you should be all right.
thanks a lot for the quick answer!
I solved the problem! it was about premiere pro, the new project i opened was a complete mess. 1280x1080 (i've no idea why), 24fps, 1.5 pixel ratio... Now i created a custom new project with the same parameters of the videos i'm editing and everything is fine!
thank u a lot again!
Premiere is interpreting the footage as 1920 X 1080 at 29.97fps with a pixel aspect ratio of 1.0. This matches the camera specs when shooting 1080p.
When you use Dynamic Link the comp size, PAR and frame rate come from the PPro Sequence Settings. Your screenshot of the AE Project Panel sows the info from the comp, not the footage. Dave missed that and thought it was showing the footage. The properties revealed in the AE Project Panel are for the comp, not the footage.
Change the Project settings in Premiere Pro from 1280 X 1080 (1.50) to 1920 X 1080 (1.0) and 29.97fps so that Premiere Pro Sequence Settings match your footage. If you're going to do any tracking it's critical that frame size, pixel aspect ratio, and frame rate exactly match the source footage. This is true no matter what you're using for tracking. Fix the Sequence settings in Premiere and try Dynamic Link again and you'll be ok.
Looks like you figured it out while I was responding.
Yep, thanks a lot anyway to everybody! You guys are super helpful! I was going mad..