If he doesn't hide that, he's liable to get smacked.
Is there not a factory reset option somewhere?
Hopefully you've learnt your lesson not to buy crappy unmodifiable junk in future? :P
As for Gmail, you can click the Details link at the bottom of the page to get a list of active sessions and sign them out - that should make the device forget your password, though probably best to test it with another device to make sure it does do that.