I am assuming that you are adding an Exchange account, since those seem to be the only type affected.
Open the gmail app, then tap on the hamburger menu (three lines in the upper left corner). You should see your gmail address listed at the top. If you tap on that you'll get a list of all email accounts that are setup (I'm going to guess you'll only have your gmail account listed) plus the options to add or manage accounts. Tap on add. From there you just pick your account type, enter your email and password and it should discover all the right settings for you. If not, you'll need to enter them manually.