iCloud mail and/or Apple ID for 90 year old mother

Yes, but that is an entirely new Apple ID, which has none of the purchases or content of other one. It's much messier since they cannot be merged or content transferred between them

Best, and easiest to just create the email address directly on the current Apple ID with the gmail address, and then change that Apple ID to use the recently created address. Note that after creation the iCloud email address, they still need to wait 30 days before they can use it as an Apple ID.

An iCloud email address in most cases is in and of itself an Apple ID and cannot be used as the address for another Apple ID.

However, Apple IDs can have associated iCloud email aliases, and those can be used to log into the account.

If the Apple ID is logged into an Apple device, you can turn on the Mail option in Settings or System Preferences for it to create an iCloud email address.

link-> Create an email address for iCloud Mail - Apple Support

With that done, the created email address can then be used to sign in to the Apple ID with the same password.

The Apple ID created with gmail services is example1@gmail.com and is signed in Mac , to change Apple ID to new iCloud mail address say example2@icloud.com change it at appleid.apple.com see the article Change your Apple ID - Apple Support

An Apple ID address say example2@icloud.com is created and connected with previous Apple ID that was signed in Mac system preferences say with gmail services example1@gmail.com it can be changed at appleid.apple.com

Suppose another Apple ID created with other Mac / iPhone example3@icloud.com is to be established in Mac , it can be done just click on edit and add it as described in the article , what asked is the easiest method .

However another method is also there but lengthy https://discussions.apple.com/docs/DOC-13561

In the screen shot sign in with Apple ID already created from another device it could be Mac / iPhone whose credentials are totally different from primary Apple ID signed in .

This is just switching Apple ID address either from signing out Mac system preferences or at appleid.apple.com

The iCloud address must already be part of the Apple ID in question to be changed like that.

You cannot just use a random iCloud account to change it. Since iCloud accounts are in and of themselves Apple IDs.