How Can I Stop Gmail Emails From Showing Up In iCloud on my iMac and iPhone?

You have created web rules on gmail account and forwarding as From say example1@gmail.com To example2@icloud.com the incoming mails from your senders will automatically move to iCloud mail address in its inbox see the article https://support.google.com/mail/answer/10957?hl=en#zippy=%2Cforward-to-multiple-accounts%2Cturn-automatic-fowarding-on-or-off

If iPhone and Mac are signed in with same Apple ID and password say example2@icloud.com pure syncing will occur .

You have disabled the gmail account in apple mail application , but the rules created in web gmail account are mirrored in IMAP account .

You can turn off mail forwarding .

Generally users don't make any changes in web accounts and keep as default , as if rules are created and you don't remember it , then its difficult to overcome , you can delete mails from iCloud.com select hundred mails or less and click on delete icon , don't select mails in thousands , however in latest version command and A works to select , avoid it as spinning beach ball will occur .