What are you doing wrong? Do you see log-affine+concave allowed in http://cvxr.com/cvx/doc/gp.html# ? It is not allowed.
You may find this post by mcg to be relevant Log of sigmoid function
Do you have a more detailed proof that gama is concave with respect to P1 and P2 than “I am sure”. None of my professors would accept that as a proof.
I will presume this problem is non-convex unless you show otherwise.
Please CAREFULLY read Why isn't CVX accepting my model? READ THIS FIRST! .