A gridNode is a giant tower filled with servers that you'll need to climb to the top of to be able to subvert it and connect it to your Beatlink - basically, when you subvert a gridNode in a district of the city, it will open up the ability to fast travel to safe houses in that district. Your first opportunity to do this will be during the Gridnode Run main mission, then other gridNodes will open up around the city and will be marked as  on your map. Simply get to the computer at the top without touching any of the lasers and this trophy will be yours. If you touch a laser, just jump to your death or restart the checkpoint, redoing the entire gridNode isn't necessary.

If you miss any gridNodes and want to replay them, they are listed as side missions in the menus where you can freely select them and play them over again.
 

How to Shut Down the First Gridnode Undetected