On a GS/GSX the only way to get the tank completely out is to remove the engine.
The good news, as Ankeneyou mentioned, is that the tank only has to be slid forward and the steering moved out of the way.
It will help to disconnect the steering cable from the arm as well. Make sure you remember to tighten that nut/bolt properly during reassembly, or you might find yourself suddenly without steering at "an inopportune time." Don't ask me how I know this