iCloud sent mail (from Gmail web interface) not listed I trying to use the Gmail send mail as feature in which I use the iCloud mail credentials and smtp.mail.me.com.

It works, I was able to send mail from Gmail web interface while the sender was my iCloud account.

But when looking for that mail in the iCloud Sent folder it is not there. So it seems there is no log in the apple side of emails sent using Gmail send as feature. Or I'm missing something?
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.