I just changed my iCloud Apple ID to a Gmail email address. This was not possible before.
I didn't know that it was not possible... but on December 18, 2019, I ordered a Simple Mobile iPhone 7 from Simple Mobile's website. When it arrived, I activated it with my existing iCloud account.(xxxxxxxx@icloud.com)
About a week later, I decided that I didn't want to associate this spare phone with my main iCloud account, so I created a new Gmail acct and changed that iPhone 7's iClound log in to this new Gmail address.... (which I've been using since then) The reason was that I used this iPhone 7 in my car, which did not have Android Auto... only Apple Car Play on the factory radio. I kept the iPhone 7 without a password in the car's center armrest and it never left the car since then.
like I said, I didn't know that it was not allowed... or was it just a random YMMV?