I have the same machine as you. As others have already said, a limescale blockage is most likely to be the source of your problems. If you have not already bought descaler then I would also recommend Puly Caff Descaler.
There are differing opinions regarding whether to use citric or tartaric acid based descalers for an aluminium boiler (as found in domestic Gaggia machines), however this is what I use and do so on the recommendation of others. (Whatever you use, I wouldn't leave the solution in the boiler for any more than 20 minutes or so). Although these instructions are for a Gaggia Classic, both machines are for these purposes the same:
http://coffeeforums.co.uk/showthread.php?1672-How-to-descale-a-Gaggia-Classic
I would also undo the two allen screws shown in your photo and try to remove the shower disc holding plate. If your machine is gunked up then it will be a real job to prise the plate away. It can take a bit of force but I carefully prise mine away with a spade screwdriver – after this clean everything you can see and then run the descaler though the machine with the plate still removed as otherwise large deposits of scale can break loose but are caught behind the plate.
The following gives you a step by step guide for removing the group plate:
http://www.bluebox.com.au/jcrayon/gaggia/
Just miss out the part about changing the gasket unless you think you need to do this as well.