I hope this is what you need:
If you‘re using the built in Mail app:
In Settings, go to Mail, select Accounts, then go to Add Account. Select Google, enter your Gmail address and password. If you use 2-Step Verification, confirm that you are trying to login somewhere else (computer, …). Then select, what iOS can access, and save the account.
If you‘re using the Gmail app:
Open the Gmail app, go to Sign in. If the app remembers the account, you just have to confirm it and allow to receive notifications. Otherwise you‘ll have to add the account.
If the problem is different, please explain what isn‘t working, and what you‘ve tried so far.
I‘ve merged both your threads about this issue. Keeping the information in one thread will more likely result in helpful advice.