What actually happens when you cancel
Canceling does not delete the email -- it interrupts the send and converts it back into an ordinary draft, with all your original text, recipients, and attachments intact, so nothing is lost.
Scheduling is not the same as Undo Send
Gmail's separate "Undo Send" feature only gives you a short window (a handful of seconds, configurable in settings) after clicking Send on a normal email. Canceling a scheduled send is different -- it works any time before the scheduled moment, which can be hours or days out.
Frequently Asked Questions
Can I reschedule instead of fully canceling?
Yes -- open the scheduled email, cancel it to move it back to Drafts, then use "Schedule send" again from the compose window to pick a new time.
Does canceling work the same way on mobile?
Yes, the Gmail app has its own Scheduled folder under the menu, and the same "Cancel send" option appears when you open a queued message there.