Change AppleID

As of the recent update you can change a non-Apple address used as an ID (such as your Gmail one) to an iCloud address which is associated with your account (you will see a list of available addresses).

When you do this the non-Apple address will be added to your account as a 'Reachable at' address, so that there is another address than the iCloud address Apple can contact you at (in case the account fails in some way).

As long as the non-Apple address is there, you can't use it as the ID for another account; you would need to remove it (and you should replace it with another non-Apple address - don't leave yourself with only iCloud addresses to be contacted at).

You could of course change the Gmail address to another non-Apple address rather than to an iCloud address if this is more convenient to you.

There may be a delay of some days after doing this before the Gmail address becomes available to use as another Apple ID.