I've had a Google account for some time. With it I manage my company Adwords account, check up on client sites using Analytics, publish my calendar, etc, etc. Running an web design business, its an invaluable tool.
Recently a friend sent me an invitation to open a gmail account, which I duly did. Everything went tickety boo. Then I was locked out of my Google account - Analytics, Aswords, Blogger, etc, were all inaccessible using my normal login. So, I whizzed off an email to Analytics support. Back came the response that I'd changed my account email address - which is also my login.
No I hadn't, not least to my knowledge. No, I had - least my gmail login is now my Google account login! This is done automatically for you it seems. That was the first I'd heard of it & I can't remember reading in the Gmail info anything that said this would happen. Perhaps I'd ticked some box without realising the 'global' implications of so doing.
Whatever, just be careful. Signing up for Gmail may have ramifications for your existing Google account!