G-Mail on iDevice?

What do you mean by "G-Mail"? Do you mean the Gmail app from Google? Do you mean a Gmail account?

I've been using Gmail for years. I don't especially like the Gmail app so I use a third-party app called Spark. I also don't like the built-in Mail app but, if you like it, you can certainly use it to retrieve mail from a Gmail account. There are lots of other apps in the App Store.

And, no, you don't need a Gmail account to use your iPhone or iPad. You can use whatever email service you want. Lots of people use the one Apple provides, icloud.com but it's up to you.

As it happens, I use my Gmail address as my Apple ID but you can use whatever email address you prefer.