You don't show any code, so my answer might be irrelevant.
Are you using diff as a symbolic derivative or as a standard MATLAB finite difference? If you are using a symbolic derivative, then ga does not apply until you convert the symbolic function to a standard MATLAB function using, for example, matlabFunction. See Symbolic Math Toolbox Calculates Gradients and Hessians.
But this brings up a more important point. If your problem is smooth, then don't use ga. Instead, use an appropriate Optimization Toolbox solver such as fmincon. Or for a global solution instead of a local solution, use MultiStart. I assume that your problem is smooth because you talk about analytic derivatives.
Good luck,
Alan Weiss
MATLAB mathematical toolbox documentation
