Sadly, on Android devices, nothing seem to work. 

The Gmail app on Android does not read the signature from G Suite so it has to be edited yourself if you wish to attach your signature to your outgoing email messages. 
Thankfully, there is a solution, even if it may seem a bit severe: Switch to iPhone! Or ask Google to provide better support for signatures in Gmail on Android devices. 

We have a feeling that for you Android users out there, this will be the preferred means of protest!

Google has recorded this feature request.

For the meantime, Please use your internet browser on your android mobile to send and receive email.