Interesting question, it's recurrent here.
You have two choices, the first one and easy, but not 100% perfect is make some invisible sprites as detectors and place them attached to your player, so, make for example 6 detectors, 3 on both sides and them an event to detect which angle is the slope and change the player angle according until the detector stop detecting the slope, also, make slopes in a group named properly to make it easy to the detector identify a slope, otherwise, your player will become crazy in any exception.
The second way, better and used for example in games like Sonic, you'll make each slope and looping with a specific ID, when the player touch it, you'll need to set an event to detect what side is him facing and change the angle according.
Look my very dust old and abandoned sample here to understand what I mean by detectors, for example:
or download this sample of Sonic and use it: https://www.dropbox.com/s/bdjlvpo2bv5cy ... .capx?dl=0