UNABLE TO REPLACE APPLE ID WITH ICLOUD ACCOUNT

Since you used a third party email AppleID to create the iCloud account, both your gmail AND the @iCloud.com address will log you in to the same AppleID. You can use whichever one you wish to use as your AppleID. They both are synonymous login IDs to the same AppleID.

About a month after you make it, you’ll be able to promote the @icloud.com address to the primary email address slot if you like. But note that once you do that, you’ll never be able to edit that primary email address for your AppleID.

IMO, it’s better to keep the third party address as the primary email address to keep the flexibility of being able to edit it later on. You can still just login to everything with the iCloud email address.

No. iCloud is it's own type of Apple ID and you cannot merge Apple ID's. You would have needed to make the iCloud ID as you followed the process to change your Apple ID email, not after .

Since Gmail is your primary, log in to your Google account and set up a rule to forward those messages to your iCloud email address. The web based Gmail account interface has a lot of features for that account.

Apple messages and updates go to the email address that is your main Apple ID. You might be able to set up account forwarding on your email provider's web site.