Using an inductive proximity switch, you might need this :
My inductive proximity sensor comes with a little board that connects to the 3 pins just near the reset button on Ramps 1.4.
You might need this too :
https://github.com/alexborro/Marlin-BedAutoLev
Now, if you successfully get all this working on your GT Prusa, please share !
I need to look into it soon.