The Gaggia Classic espresso machine was first introduced in 1991 and soon became a very popular home espresso coffee machine.
The Gaggia classic aluminum boiler was cheap to produce and heated the 80ml boiler water very quickly - so efficient that the aluminum boiler is still available on the new Pro models even today. A clever design with 2 external elements to prevent the boiler from burning out. The 3-way solenoid valve allows for a backflush and prevented the coffee from sneezing - (coffee exploding from the portafilter after steaming).
The OPV (over-pressure valve) allowed users to adjust the 9 bars of pressure to suit different coffee types.
Although standard models came with the Panarello steam wand - it was easy to upgrade to a Rancilio Silvia steam wand, for fine latte foam. Because the Gaggia was made so long ago and is so popular parts generic non-patient parts became widely available and this has led to its popularity.