The Gate agents see when you have checked in, what your seat assignment is, if you have a special meal or request, what fare you hold and if you have the passport information entered etc. Also they can see the seatmap of the aircraft and if there are seats that were reported broken on the inbound flight so that they can re-assign the passengers to better seats or give vouchers for that.

Also the agents can see any delays and even have ways of getting the EasyInfo Gate Displays to show the flight information.

They might even be able to log onto Flyertalk to see if you are a regular poster lol.