A keyboard is required. Shortcuts work only with a hardware or Bluetooth keyboard attached to the iPad. There is no setting to enable, unlike Gmail on the web.
Core actions are single keys. c composes, r replies, a replies all, f forwards, e archives, # deletes, and x selects.
Composing uses Cmd. Cmd+Return sends, Cmd+Delete removes a line, Option+Delete removes a word, and Cmd+W closes the message.
Navigation. j and k move to the next and previous conversation, n and p move between messages, Return opens an item, and Escape goes back. The full set is in the table above.