If you are using iphone and you had saved the files to your photos app on the phone then you can get the media by connecting your phone to itunes. If on android then you can connect your phone to your computer using USB and navigating to the photos folder and downloading the video from there.
The quality will be good. I just did a test run and was able to download a trimmed clip at 1080 resolution and 45Mbits/s data rate. The saved clips are not low res copies. Also they are MP4 and can be imported into GoPro Studio no problem.
yeah i just downloaded it and imported into GoPro Studio. it was 28MB file and only 5 seconds resolution 1920x1080 and 45Mbits/s data rate.
It needs to have been saved to the phone though.
Also if the video was uploaded to youtube at any point, it can be downloaded from there by going to the video manager (https://www.youtube.com/my_videos) and click on the edit button next to any video and selecting download MP4.