There is a product called cafetto that you can get from coffeeparts or any good espresso machine suppliers or the site sponsors.
A bottle will cost you about $24 depending on where you get it but it will last you a long time. Follow thge instructions on it. Basically you just run it through the machine followed by flushing plenty of water through.
Whatever you do - do not use citric acid on your machine as Gaggias have aluminium boilers and citric acid will kill it dead.
I hope this helps