When I press some email-link on Safari, it goes to mail app. How can you make it go to Gmail app?
At the moment, unfortunately, Apple doesn't allow default apps to be changed.
Some apps came up with various hacks to open Google Chrome instead of Mobile Safari when you press a web page link within that app but at the moment there is no such hack for mailing. And if there was, frankly, Apple would't implement it into Safari themselves.
Maybe in the future, Apple will allow default apps to be changed but basically you're stuck right now. Sorry.