If water still coming through your steam arm it could be the outlet valve blocked, when cold and un-plugged, remove the shower disc and the holding plate. A 13mm socket will remove the valve, I it comes out whole that’s perfect, if you only get the bottom part and the spring and bung dnt panic, its ok. Either way use something similar to a paper clip and poke the outlet on the brewhead to remove the blockage. If the valve is whole you will also need to split in 2 and clean out. Pump water through, making sure that you have paper towels around as it can splutter. You should see residue in the container. Replace everything in a reverse way then hopefully you will be working. If still faulty seek a gaggia manual service engineer.