What is the difference?
I had the feeling that @icloud.com had to replace @gmail.com but then realise that I still needed my gmail account in my iTunes account (iTunes won't let me put my iCloud).
It's just confusing.
iPhone 4S, iOS 6.1.1
@gmail.com and @icloud.com are not related to each other, being provided by different companies.
You have evidently used your Gmail address when setting up your Apple ID, and then created an iCloud account with its own @icloud.com address.
You can change your Apple ID from the @gmail.com address to another non-Apple address which hasn't been associated with an Apple ID; but you cannot make your @icloud.com address your ID.
@gmail.com and @icloud.com are not related to each other, being provided by different companies.
You have evidently used your Gmail address when setting up your Apple ID, and then created an iCloud account with its own @icloud.com address.
You can change your Apple ID from the @gmail.com address to another non-Apple address which hasn't been associated with an Apple ID; but you cannot make your @icloud.com address your ID.
iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support
Although you can make the @icloud.com address your ID when you first create an ID and an iCloud account together, once you have created an Apple ID using a non-Apple address and then created an iCloud account, you can't make the iCloud address into the ID. I don't know exactly why this is - it may be a technical issue with how IDs work but Apple don't explain these details to us mortals.
Thanks Roger, that helped (by making me stop wanting to switch all by accounts from @gmail to @icloud). For my general knowledge, why can't I make my @icloud.com address my Apple ID?
Difference between email @icloud.com and @gmail.com?
