So Google hasn’t ironed all the kinks out yet for their help and support regarding the newly rolled out HTML Signatures, as this simple fix alluded the Google Support team for a week.
If you’re having trouble finding out why your GMail won’t show an HTML signature, try this:
Switch the Email Composer to Rich Formatting, instead of Plain Text.
1. Click compose mail
2. Click on “Rich formatting” (under Subject and just over the message textbox)
3. Go back to settings (no actual message has to be created, this is just to change that setting).
Why this isn’t simpler in the settings, I don’t know.