Below are some links showing you how to install an HTML email signature in various email clients:
Gmail on iOS Devices (iPhones/iPads):
For Gmail on iOS devices, as long as it’s properly installed in Gmail on the web (per above instructions), just make sure that “mobile signature” is turned off in the Gmail app on your iPhone/iPad by going to Settings > Signature Settings > un-tick “Mobile Signature”.
It won’t show up when you’re composing a message on your phone, but it will pull your signature from where you originally installed it in Gmail and send it along with your message.
Gmail on Android Devices: https://youtu.be/OrqF_pOs560