Selection panel

I have this grid where when the user clicks on a square a prefab is instantiated, I want to make it so there is a selection bar on the side - made up of GUI buttons - and when you click on a button and then click on the grid it will instantiate a different prefab.

You could use a temporary gameobject variable that stores the current prefab that you want to instantiate, initialized to some default prefab. Change it to the according prefab when clicking on the buttons.