He isn't asking to merge accounts. He is asking if an @gmail account can be changed to a new @me iCloud account. Old mobileme @me accounts cannot be altered.
No one seems to really know at this point whether iCloud @me addresses operate the same way as mobileme or not unfortunately. I have been wondering this as well.
My first sentence about merging was just confirming what he said but that it applies to iCloud too. The rest of what I said answers what he asks, which is if he can have an iCloud email with his Gmail apple ID, I was just informing him, from my experience with iCloud so far, you can make an iCloud email address as part of your existing apple ID and that you don't need to change the apple ID.
When you go into the iCloud section in settings after installing iOS 5, you log in with your Apple ID and when you turn on email, it asks you to choose a me.com email address and sets it up for you in mail. Your apple ID doesn't change. With iCloud on the web I log in with my Apple ID (myusername@live.com) and i get the mail web page with my me.com email.
iCloud control panel for windows does the same. I have iCloud email and keep my existing apple ID login address.
My point is that you don't need to change the existing account like you said, you just add to it or if you want to fully change, go for it