How do I get all my archived Gmail into iCloud mail?
Find a IMAP email client, like Thunderbird.
Configure both Gmail and iCloud mail as IMAP email accounts in Thunderbird.
After setup, you'll find both of you accounts in the left pane. Now you can drag and drop from Gmail to iCloud. Be patient as the process might take some time.
I haven't tested this with Thunderbird lately. You probably can use Apple Mail for this also.
A different way is to use Microsoft Outlook, configure both iCloud and Gmail. Let Gmail sync to the finish. Then you can export the mailbox and import it again into iCloud mail.
Exporting: https://support.microsoft.com/en-us/office/export-or-backup-email-contacts-and-calendar-to-an-outlook-pst-file-14252b52-3075-4e9b-be4e-ff9ef1068f91
Importing: https://support.microsoft.com/en-us/office/import-email-contacts-and-calendar-from-an-outlook-pst-file-431a8e9a-f99f-4d5f-ae48-ded54b3440ac