What if I already have a Gmail account?

There are a number of things you could do:

Option 1: Log out of your personal account and then log into your TigerMail account: https://support.google.com/mail/answer/8154

Option 2: Log into both your Tigermail and gmail account at once: https://support.google.com/mail/answer/1721977?hl=en

Option 3: Open a new "incognito" window and then login to TigerMail: https://support.google.com/chrome/answer/95464