The new auto-fit feature will let users enjoy the pinch-to-zoom gesture within emails. This function has not been enabled by default and there is no obvious option in Settings that will lead you to enable pinch-to-zoom. To enable it, you will have to go through the following guide:
Install the Gmail 4.2.1 app update from the Google Play Store. If you have it installed, just open the app.
Press the Menu key to go to the Settings of the Gmail app.
From Settings, select General Settings.
Scroll down till the Auto-fit Messages option and select the empty box to activate it.
After this, you can use pinch-to-zoom gesture on any email you receive from this point onwards.