How to overload Gdn_Form

If I want to overload a model or controller, I can typically do this in a hooks file:
Controller_Method_Create()But how can I apply that to Gdn_Form or other "core" classes? Doing this:
Form_NewMethod_Create()doesn't work, and I've tried a number of permutations with the Gdn_ prefix, etc. Could I get the Cliff's Notes on this one?

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Is there something you want specifically that I can help get in?