There are two possible reason as to why your video looks so different in PPro than in what I assume is your media viewer of choice. The first reason is that Premiere Pro will downres your media while you’re viewing your edited sequence so that it doesn’t suck up all your processing power just to preview some changes. The second reason may be that since you aren’t altering your source footage before it comes into PPro, you may be editing in the H264 codec that your GoPro shoots to. This is not an easy codec to work with in most NLEs. Most users transcode or convert their footage before bringing into the NLE so that they can edit with faster responses and better video quality. If you’re using a Mac, I would suggest using my company’s new tool, Edit Ready. Edit Ready allows you to quickly and easily transcode your source footage before beginning your edits, in any of the major editing codecs. You can test this workflow solution by downloading a free trial at https://www.divergentmedia.com/editready
Hope this helps.