I descaled my boiler when I got the Gaggia. I used two bottles of Durgol descaler that seemed to get the job done, but when I give the steamer a few minutes longer I seem to get more power. I wonder if it isn't time to open up the boiler. I opened the top up before, elegantly simple interior workings. If I have the time, I might give the boiler a thorough descaling or replace it. Though, the $75 for a replacement, might make it not as good of a deal.
If I take the Gaggia apart, I'll post a few pictures because I'm guessing it is a pretty popular machine.
How long do you let the Gaggia warm up? I got better steaming results after waiting 20 minutes.
I've been having MUCH better results letting it warm up about 10 minutes after flushing the boiler a couple times with the brew button. The only down side to this is I have to pull my shot after steaming milk and clearing the steam from the boiler. I might need a little more aggressive descale, but waiting a few extra minutes beats cracking open the boiler anyday.
This guy has a pretty good shot by shot disassembly of the Gaggia boiler, if anyone is interested:
http://protofusion.org/wordpress/2012/04/gaggia-classic-disassembly-and-cleaning/As a complete aside, if you haven't adjusted your OPV, I would highly recommend it. Mine was at 12 bar and the adjustment took my espresso from mediocre coffee shop quality to top level.