How can I add an email signature in my iPhone Gmail app?
Open Mobile Safari and visit m.gmail.com  
Sign in and navigate up to the list of folders.  
There's a gear wheel in the top right, tap it.
In here you can set your 'mobile' signature and opt to use it instead of your desktop signature.  


This will now also apply to mail sent via the Gmail app.  

EDIT

Theres an update to the gmail app that allows you to do the same thing but from within the app.  Show the menu by clicking the icon in the upper left of the screen and you'll see a gear wheel.  Tap that and you can set your mobile signature in there.