Running a program/script before a user logs in on RedHat Linux GUI

I am looking to run a script prior to a user being able to log in on the GUI. Its a basic script, do you accept Ts&Cs Y/N, enter user name, data is captured and stored. Then once complete takes them to the log in screen. My issue is I am unsure where or how this would run (being relatively new to Linux and scripting). I have searched and played about with gdm in terms of just displaying a user agreement when logging in but I would like to create something the user has to interact with. Any pointers on where and how to start would be appreciated.