Old ID (gmail) to new iCloud I’d keep info
How do I merge my previous Apple ID (gmail) with my new Apple ID (iCloud)
iPhone 6s
You cannot do so. You cannot edit an @icloud.com, or @mac.om or @me.com email address when in use as the primary email address as an AppleID. And you cannot merge AppleIDs.
Why did you make a new icloud Appleid? Do you have an iCloud account associated with the gmail based AppleID?