Introduced in iOS12, Siri Shortcuts are a simple way to group a set of iOS actions. The actions are then triggered using Siri voice commands.
Shortcuts work with the majority of inbuilt iOS apps. Over recent months an increasing amount of third-party developers, such as Spotify and Google again with their Maps app, are introducing Siri integration into their software.
It has taken a while for Google to implement Shortcuts in Gmail. Plus, at this point, it only includes one voice command, the ability to send an email.
You can activate Shortcuts in the settings of the Gmail app, or from within Shortcuts itself.