iCloud sent mail (from Gmail web interface) not listed
SMTP servers don’t always save a copy of outgoing messages in a Sent folder. Gmail’s SMTP server does, but iCloud’s leaves that behavior up to the mail client (in this case, Gmail's Send As feature).

When using an external SMTP server, it appears that Gmail does not instruct that SMTP server (in this case, iCloud) to store a copy of outgoing messages in its Sent folder; that is why you don’t see the record you’re looking for in iCloud.