Gmail for iOS received an update that includes the ability to open links in Gmail directly to YouTube, Google Maps, and Chrome, assuming the apps are installed.
The feature can be turned off if you prefer.
What’s New in Version 2.2.7182
– YouTube, Google Maps and Chrome links go directly to the app instead of the mobile web, if installed. You can turn this off using the in-app settings.
– Added support for signing out of a single account instead of having to sign out of all accounts at once.
The update is available now in the App Store.