Finally i decided to use an ellipsoid for my character/world collision detection. I have the radius eRadius(x,y,z) of the ellipsoid. I translate all the map vertices to the vector space defined by eRadius (so, the model is bound to a sphere of radius 1). Collision works well, but the problem is implement gravity, i dont know how to do so. Model has vPosition, vVelocity and vRadius properties (all 3D vectors). BTW in the collision detection function i can save the floor position (polygon with normal(0,1,0) that is below sphere). How can i implement gravity? Thanks

Gravity (and other forces) can be represented simply as an acceleration vector, which you apply to your velocity vector once at the beginning of your collision/movement routine.

Here is a good article which may help you out...