You can descale Gaggia coffee machines by filling the water tank and running through descaler solution, but it is far better to remove the boiler so the extent of the scale build up can be seen.
Any solution capable of removing scale will case some damage to the metal it comes in to contact with. By disassembling the machine, the descaling solution can be in contact with the parts for the minimum amount of time.
First, disconnect the power and remove the lid/funnel arrangement by removing the two screws at the rear of the lid.
Once inside, it is recommended to mark the wiring connections to the thermostats and boiler elements. You can see the connections to the steam thermostat are marked with black dots, and the element connections have been marked with stripes.
With the wiring marked up, proceed to remove the steam knob.
Remove the milk frother.
Remove the four allen screws securing the boiler in place.
The boiler can be removed as one piece once the wiring has been disconnected.
Split the boiler and replace the o-ring seal.
Upturn the boiler, and fill it with descaler. Depending on the strength of the descaler, and the extent of the scale, it can take anywhere from 20 minutes to several hours for all the scale to be removed. As soon as the descaler has completed its task, pour it away and thoroughly rinse away any residue to prevent further chemical action.
To clean up the group head, remove the shower screen centre screw.
Beneath the shower screen is a plate held in with two allen screws.
Remove the plate.
At this point the steam valve can be seen. This is often blocked by pieces of scale, or the rubber becomes brittle, either way it results in leaks.
This also a good point to replace the group seal.
At this stage, I would clean the valve and hope that a leak-free seal results.
If you need to replace the valve, a large flat blade screwdriver will be needed to remove the body of the valve from the group head.
Following reassembly, thoroughly flush the system. I would suggest running a full tank through before making any coffee.