Apple ID

You are welcome. I would leave your Apple ID as the Gmail address for the reasons I listed above. If you permanently change it to the iCloud address, it limits your options if you need to change it in the future.

The second is an alias of the first. And, you can sign into all places requiring an Apple ID/Password using it. See the related information here: Change your Apple ID  https://support.apple.com/en-us/HT202667. Read this section: If your Apple ID is a third-party email address.

The @iCloud address is an alias of your Gmail address.

You can use either one for your Apple ID. Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

Based on what I have read, I suggest you leave your Apple ID email address as a 3rd party email address because you have more flexibility if you need to change the email address. For example, if the Apple ID got compromised, you can change it to another address. I also have read of many people who were locked out of their accounts when they forgot their password. Since the recovery email address is the @iCloud email address, if you forget your password, you also can't access your email to get emails from Apple to recover your account. You can set a rescue email address, but for some people, this hasn't worked.

You are welcome. My pleasure.