The best (and easiest) way to add an email signature to your Gmail App on Android is to do it by using the web-based version of Gmail on your computer. When you do that, your email signature will automatically be appended to all your emails sent out from your Gmail App on your Android. This is done by the server-side processing of your emails on the Gmail servers. Simply log in to your Gmail account, and install the email signature through there.
When you do this, make sure that your mobile signature is turned off in your Gmail App on your phone. Otherwise, it may conflict with the one you set up in webmail, and may not work correctly. To turn mobile signature off in the Gmail app, go to Settings, Select your email account, Signature, Set the Signature to “Not Set”. To do this, you need to ensure that the signature text box is empty and then it should say “Not Set”.