You may have accidentally changed the amount setting for your espresso shot when resetting the descale counter.
If you lost the manual you can find one here. http://www.gaggia-usa.com/support_docs/Gaggia_Brera_Quickstart_Guide.pdf
On page 11 (with the printed number 31) you find how to change these settings:
• Place a cup under the dispensing spout.
• Hold the "espresso size" button pressed for 5 seconds to enter the programming mode; the "coffee" icon will blink quickly and the coffee will start to be brewed; the icon will flash during the whole coffee brewing cycle.
• Press the same "espresso size" button again when the amount of coffee in the cup reaches the desired level.
At this point the button "espresso size" is programmed; each time it is pressed and released, the machine will brew the same amount of coffee that was just programmed.
The same operation can be repeated for the "Lungo size" button.