The Gmail app on iPhone has no HTML signature field of its own. It uses the signature you save in Gmail on the web.
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Set it up in Gmail on the web
Follow the Gmail guide on a computer. Once you save, it syncs to the Gmail app.
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Turn off the app’s mobile signature
If you previously set a mobile signature in the app, turn it off so it does not replace the web one. In the Gmail app go to Settings → your account → Signature settings and switch the mobile signature off.
You will not see the signature while you compose in the app. It is added when the message is sent. Give it a little time to sync.