While GlobalProtect's login screen asks for your email address, this is misleading and a known issue with the login screen.
Using your email (typically first.last@colostate.edu) will result in a failed login even with a correct password.
Instead, globalprotect needs your EID/NetID for login, followed by "@colostate.edu."
If you mistakenly entered your email (first.last), you can change the login credentials from the password screen by clicking "edit" above the password field.