Following recent changes to the Gmail™ mobile apps, you can no longer use the signatures you've set up on desktop for mobile email correspondence.
Signite Mobile Extension, a complementary app for the Signite signature management app, lets you do just that. 
Manage and deploy your signatures on Signite and use the Mobile Extension app to apply them to your Gmail™ messages directly from your mobile device, without any additional hassle.
Note: This app requires that you first install the Signite app and create an account on signite.io