Unfortunately, I don’t think the Gmail app (or any of Google’s apps, for that matter) support Shortcuts.
As @ismh can attest, it’s a running joke on the Connected podcast to see how long it takes Google to support basic features of iOS such as split-screen (and now multiple windows). I can’t remember the details (maybe Stephen does) but when iOS ¿11? came out, it took something like 9 months for Google to add support for the new features to their apps.
(And IIRC even then I think it only happened after Google got caught mis-using their Enterprise Certificates for iOS the same way Facebook was using theirs to offer its own VPN, so it may have been Google trying to “play nice” with Apple, or it may have been a strong “suggestion” from Apple that they should finally get off their *sses and do it.)
A cynical person might suggest that Google wants their apps to be “better” on Android than iOS so they are in no hurry to improve their iOS apps beyond the basics. I’m not sure I believe that, but the thought did cross my mind, and I suspect that if these features were on Android, Google’s apps would have day one support for them. So… I guess maybe I do believe that.