How do you send gmail canned responses on an iPhone?

I am on the hunt for an answer to this question myself.

As far as I am aware so far, using a text replacement keyboard shortcut (Settings > General > Keyboard > Text Replacement). Here you can add that text, and then designate a shortcut to trigger that text. I've used it in many scenarios, and is helpful, but will be cumbersome to set up (I've got 20 canned responses I'm trying to set up).

Hope that helps.

Are you referring to some type of preformatted mail message? I am not aware of anything, unless you take the time to search the app store. Unless there is some type of setting in the Gmail app.