You can delete the message by its Gmail	ID.
$ gam user testsimple show messages  showlabels
Getting all Messages for 
tests...@domain.comGot 1 Message for tests...@domain.com...
User: 
tests...@domain.com, Show 1 Message
  Message: 19a2741938d1e79d
    Date: Mon, 27 Oct 2025 13:00:00 -0700
    Subject: Scheduled
    From: Test Simple <
tests...@domain.com>
    Reply-To: 
tests...@domain.com    To: Ross Scroggs <
ross.s...@gmail.com>
    Content-Type: multipart/alternative; boundary="0000000000007b8679064225688c"
    Message-ID: <CAGo8KJ8WfVG0bMZC81tGPtTpK8pUmXQsfhPbiPtRs7oLV8vu=
w...@mail.gmail.com>
    Labels:
$ gam user testsimple delete messages ids 19a2741938d1e79d
User: 
tests...@domain.com, Delete 1 Message
  User: 
tests...@domain.com, Message: 19a2741938d1e79d, Deleted
The trick is identifying the message, I	don't see a query that identifies scheduled messages.
You'll have to find it with another filter, subject for	instance.
Ross