If you are a G-suite user then the signature is automatically installed on your Gmail mobile app for IOS.

Google doesn't show the signature when you compose a message but it does actually appears once you receive the email :)

If it still doesn't work make sure that the signature option is turned off in your app settings. Like this

For Gmail Android app, unfortunately, Google doesn't allow signature in their settings. It's not something we have leverage on. The developer community has been complaining for a while now but nothing changed so far.