Please read Why isn't CVX accepting my model? READ THIS FIRST! .,
Have you read http://cvxr.com/cvx/doc/gp.html and specfrically http://web.cvxr.com/cvx/doc/gp.html#constraint ? Obviously, you are violating the rules presented there when ‘>=’ is refused by CVX. In such case, have you proven the model is convex? if you want to understand the CVX GP rules are a deeper level, and get insights on possible reformulations, please read https://web.stanford.edu/~boyd/papers/pdf/gp_tutorial.pdf .