If the question were" When can you overide a GPWS warning" that is completely different to " when can I ignore a GPWS warning".. Nuisance warnings do occur, and NO WARNINGS are ignored, however, warnings can be overidden. Examples would be circle to land with terrain inhibit, non-normal landing configs, flap inhibit and so on. As a rule, you can only overide the warning if you know why it has occurred or you are visual, or both. The non-normal sections of both the Airbus and Boeing FCTM tackle this issue.