It is possible to use Gmail email in an iPhone or iPad and once you have done it once, you will be able to do it without a problem as many times as you want.
The first thing that you need to do is to check if you already have the Gmail app in your phone. You can do so by accessing the menu, where all the apps are stored.
If it is not there, you will need to download it. Do not worry, this is a free app and as it has been developed by Google, it is reliable too.
Go to the iTunes app store. You will have an app in your iPhone or iPad for doing this. Click on the app and it should take you to the iOS app store, where you can select and download apps of all type.
Search for the Gmail app and download it. It should be free of charge and it should have been developed by Google (check this to make sure you are on the right app!).
Once it has been downloaded, click on it and it will open. Of course, you will need a Gmail email account to be able to use it, and you will only need to introduce your email and password once for Gmail sign in email.