Do you mean the native Mail app?
If so, you can add more than one GMail account under Settings > Mail, Contacts, Calendars > Add Account.
Tapping the MailBox icon at the top left of the mail list (in landscape view) you will go back to a list of Inboxes and Accounts. Using this you can switch between your GMail accounts instead of seeing it all at once.
If you are talking about using GMail in Safari, then you should see your email account name below the mail list. Tap on this for options to sign in and out of accounts. You can even be signed into more than one account at a time, and switch back and forth. At least I think you can. Only having one account I can not actually test this.
And then there is the GMail app in the App Store. It's similar to going to GMail in Safari feature wise, and has an explicit Sign In/Out button below the email list/column.