If all the video is there, it's easy: conform the frame rate. Go File>Interpret>Main, and change the frame rate. North America, 29.97 is good, for Europe, 25 is good.
The lower the frame rate, the slower the motion.
AE cannot do anything with this meta data. It only sees the discrete frames and the start and end markers stored in the file. Anything else will have to be done manualyl using time-remapping and setting the correct footage interpretation.
Most newbies don't understand slow mo and frame rates at all. I have shot footage from 1 frame every half hour to 20,000 frames per second.
Here's the deal. If you shoot 120fps with your GoPro and you want it to playback in real time you either have to skip some frames or find a device that will playback full sized video at 120 fps - and not many of those devices exist. If you want to playback in slow motion then you playback your footage at a normal frame rate. Let's say your film is going to be NTSC standard 29.97 (nearly 30) frames per second. AE will probably interpret your footage as something other than 120 fps. The maximum interpreted frame rate for AE is 99 fps. This may sound like a limitation but it is not. I have processed a fair amount of forensic footage shot at 20,000 fps in AE to show how wing spars break and hydraulic lines fail. The trick is that we wanted the footage to playback slowly so we could see what is going on. My solution, set the footage interpretation to 29.97 fps. If you interpret your 120 fps GoPro footage at 29.97 fps and then drop it in a standard 29.97 fps comp 1 second in real time will take 4 seconds to play back. 120/30 = 4.
If you have a need to have the footage playback in real time then you keep the interpretation at 29.97 fps and speed up the playback using any of the methods available in AE. If you choose time remapping then you'll get a keyframe at the head and tail of the shot. Let's say your shot in slow motion took 8 seconds to playback. To make it playback in real time you simply do the math and move the last keyframe to the 2 second mark. Now the 2 seconds of real time take 2 seconds to play back.
I hope this helps. As long as you understand frame rates, interpretation and playback frame rates you can handle any footage at any frame rate. You just have to do the math.