If Gradient Descent gets initialized in such a way that it starts at a local maximum (or a saddle point, or a local minimum) with gradient zero, then it will simply stay stuck there. Variations of GD, such as Stochastic GD and Mini-batch GD try to work around this by adding an element of randomness to the search, but even those aren't guaranteed to escape a zero gradient region if the shape of the gradient is weird enough.
In practice the only way to solve this is to reinitialize your search with new weights or parameters that start in a completely new region of the search space. This won't be hard to do, since if you do get stuck in such a zero-gradient area you will notice very quickly that the error in your training isn't changing at all, and you would know that you need to start over.