If the GPS is enabled (in the notification view at the top, the GPS icon is enabled), but the app doesn't use it, does it drain battery? I understand that GPS drains battery because it requires a different component to be charged up. Or does the fact that the GPS is enabled only mean that the App can use it?
The GPS icon in the status bar means that an application is currently using the GPS so it is draining the battery. GPS does not however drain any battery just because you have it enabled in the settings menu.
Technically, the GPS icon is shown whenever the GPS chip is being used. If it isn't being used, it won't be shown.
Poorly designed apps may however keep trying to use the GPS feature just because it is enabled and thereby drain your battery. Example - keeping one's phone inside one's pocket or basically without having sight of GPS satellites will cause the GPS chip to keep searching thereby draining the battery.