Although it is often referred to as “Google Authenticator” this is just a shorthand way of referring to a software standard Time-based One Time Password (TOTP) .  There are a number of different programs, including those that run on Windows and Mac computers, which can generate the 6 digit code.  In addition, there are other methods, including downloading a set of one time use codes, which users can print out or save on their desktop.  The options are discussed here: https://login.gov/help/get-started/authentication-methods/.