GMail app not displaying signature
Based on the suggestions given by Gmail Help (website) forum  just double these issues:


Make sure you are not using an alias email to send your emails
Usually the signature does not show when drafting a message but is appended when sending (try tapping 3 dots at th bottom of message and check
if see the signature)
Sometimes its the way the app is designed:



  The signature does not show during drafting a message it is appended
  upon sending.
  
  Try checking  in your "Sent Mail" and if you don't see the signature,
  there are three faint dots at the bottom of the message, click that
  and you will see the signature.


Sources:


Why is my inbox signature not showing in my email after I send
Signature hidden by "three dots"
Adding a signature to reply and forward emails