Pour the whole bottle of Gaggia descaler solution (250ml) into the coffee machine's water tank then fill the tank with fresh water.
Remove the shower disc ( it has 1 screw in the middle) and behind the shower holding plate (it has 2 hex screws), give them a good clean and make sure all holes on both are free of dirt (do not use a pin to clean the filter).
Turn on the machine and dispense 2 mugs of water through both of the steam wand and the brew head.
Switch off your coffee machine and let the descaling solution sit for 20 minutes. Never let the coffee machine heat up while you have descaling solution inside tis water system!
Switch the machine on and dispense 2 cups of water (150 ml) through both of the steam wand and the brew head and then Switch off the coffee machine for 3 minutes.
Repeat Section 5 until there is no descaling solution left in the water tank.
Rinse the water tank with clear water and fill it up again.
Rinse the coffee machine's water system with 4 liters of clear water through the brew head and the steam wand. Do not run the water pump continously! You need to stop for sometimes.
Run some water through when the machine reaches its coffee producing temperature
Srew back the shower holding plate and then the shower disc.
Do not use descalers contain formic acid (only citric acid)
Never use vinegar! Here you can read, why don't use vinegar to descale coffee machine.