For my current project, I need to make it so that the player can go higher or lower depending on how high or low the heightmap is. However, the Mode 7 engine I'm currently using does not support that at all.
I assume I'll need to change the gravity in order to make this work. How do I do so?
It's entirely possible to adjust the vertical position of an object on screen in relation to it's position on a height map. It just depends on how the data is stored.
If you can retrieve the height value of a certain spot on the height map then all that's next is to decide where the object is located on the plane relative to the height map.
Of course one needs to know how the engine that you are using works first to determine that.