If you already setup your Gmail signature in browser (Google Chrome, Edge etc.,) unchecked the Gmail app signature in your mobile device. Make sure it is off since having this on will override the browser Gmail signature.
The signature is not visible when you compose an email in your mobile device but once you send your email, the signature will incorporate automatically.
Important: Gmail app keeps on updating and the signature sometimes is not responding. If you still encounter the issue after the process above, kindly check the Gmail app update online.