You might want to clarify your post. Do you mean that you can only view your @gmail account on the web and not in mail, or do you mean that your mail is not going to your apple address (@icloud, or @me) account.
If you want to view your gmail in the mail app, the setup is straight forward. If you use 2 step verification, you have to create an app password in gmail and use that instead of your log in password. Click on your gmail address in the top right and go to account, and then security. If 2 step verification is on, go to app password settings and generate on for your mail app. Just copy and paste that password into your Mail app.
If it's the latter, when you signed up for gmail; you may have turned on mail forwarding; again this is simple to change. Go to settings, and then accounts. If your other account is listed, just delete it as it's forwarding your mail to your gmail account.