hey, you will need to probably look into height maps. This system only contains very basic collision detection (non-rotated bounding box). One thing you could do (however) is possibly adding a "wedge" type of model and depending on where the player is at on the "wedge" you could adjust the z position. Ultimately though it would be easiest to make/generate a heightmap. :) I hope this helps! I might possibly include them in future updates, but the software is sold as-is and there are no guarantees that this will be included.
EDIT: Here is a great article about heightmap generation!
**Note this tutorial is for an older version of GM but the same concepts can still apply**