Semidefinite programming

how to write this in cvx?

See my answer at Eigenvalue in cvx , and then put in the residual effort to handle this question.

but i want to find x where G(X) is minimize

variable x(n)

then form G

minimize(lambda_max(G))

That will constrain G to be psd.

can you help me in writting G(x)?

You need to form the matrix G(x) in CVX the same way you would if x were a MATLAB double precision vector. That is a matter of MATLAB syntax, not anything unique to CVX.