An explanation with a capx.
I also had made this image to illustrate the principle in another topic:
<img src="http://dl.dropbox.com/u/36472942/construct/forumhelp/LayerRotation.jpg" border="0">
The black big rectangle is the layer unrotated, the black crosses are image points of the unrotated object, red big rectangle is the layer rotated and the red crosses are the "for C2" positions of the imagepoints, whereas their coordinates (x, y) are unchanged.
So for gravity, it is the same, it is a force applied to a certain angle (towards the bottom of the screen).
If you want to handle gravity with a layer rotation, set the "regular" gravity to 0, and everytick apply a force to the object at RotatedLayerAngle + 90. (example capx)
Edit: Physics and other behaviors are not meant to work with each other.
Either stick to platformer/solid (same rotation principle applies normaly and I believe you can set gravity on the platformer behavior to the angle of your choice) or use only physcis behavior (platforms being immovable physics objects).