The Gmail app on iPhone has no HTML signature field of its own — but it will automatically use the signature you set in web Gmail. So the easiest approach is to set it up once on desktop.
1Set up your signature in web Gmail
Follow our Gmail (web) guide to add your Mirosign signature on desktop. Once saved, it syncs to your iPhone’s Gmail app automatically.
2Turn off the app’s mobile signature
If you previously set a Mobile Signature in the Gmail app, turn it off so it doesn’t override the web one: in the Gmail app go to Settings → your account → Signature settings and switch the mobile signature off.
Note: you won’t see the signature while composing in the app, but it is added to messages you send. Allow a little time for the change to sync.