I think you've pretty much identified why it's a waste to get a manufacturer/dealer who's concerned about liability to suggest home users perform the descaling maintenance. Perhaps I can help you here, though I am not sure if I can publicly distribute the document, so... check your e-mail. More generally, I would ask you what's wrong with the Water FAQ guidelines?
Water FAQ wrote:Generally, a flush through descaler uses about .5 to .75 fluid ounces (1 to 1.5 tablespoons, or 8 to 12 grams) of citric or tartaric (grape) acid powder dissolved in 1 liter of water. This is a 2.25% to 3.5% solution, equivalent to 33% to 50% dilute lemon juice. Cleancaf and other coffee manufacturers' descalers use this formula. Theoretically, these amounts will dissolve about 12 to 18 grams of scale per liter, but that would require leaving the solution in for several days; in practice, it is used for an hour or two to dissolve up to 5 grams of scale.
The formula is mild enough to be harmless to espresso machine components, but it will come out of brass or copper machines with a slight greenish tinge. This comes from milligram levels of dissolved copper and is no cause for alarm.
The FAQ suggests ~1 tbsp/L concentration of citric acid powder, and suggests leaving it in the boiler for about an hour. More than that is plenty safe; Citric acid is not a superheavyweight acid as they go.
Basically, with the GS3, shut the machine off, drain both boilers, close them back,put the pump intake into a properly formulated (and FULLY DISSOLVED) solution of citric acid, and turn the machine back on. Make sure that if you're using a small reservoir like the internal one that you have spare formula on hand, as it will take more than a full reservoir to fill both boilers. Remember to bleed the brew boiler as per the manual instructions. After the boilers fill, disconnect the level probe on the steam boiler and listen for the autofill to activate. Let it run for a few seconds (5? just a guess) to get the boiler level higher than normal, and plug it back in. Now, you can let the machine heat up, and then draw some water through the group, water tap, and steam wand every few minutes. The idea there is to make sure the acid gets to all the places where you may get mild scale formation, including the water tap, steam pipe, and the 3-way valve. After probably 30 minutes, it's a good idea to turn the machine off and let it cool down for awhile. Letting it go for another hour or more rather than 30 minutes is not going to cause any damage to the machine, and if that's what it needs to cool off enough for you to feel comfortable accessing the drains, you should do that (alternately, if you don't want to let the machine heat up, you don't have to--but the heat makes the citric acid act faster and also lets you push it through the steam wand, which you wouldn't be able to do with a cold machine).
After the machine has sat long enough to be safe to get at, empty any remaining acid solution from the reservoir, rinse out the reservoir, and fill it with fresh water. You may want to turn the machine back on for a moment and draw some water through the group, as this will flush the brew water lines of citric acid water and may reduce your flushing need. Turn it back off, drain the boilers, close them back up, and turn the machine back on to refill (again, bleed the brew boiler). You may want to let the machine heat up again and flush fresh water out of the taps/steam wand and brew group. Again, you can get at least the water tap and the brew group if you don't let it heat up.
Repeat the steps in the last paragraph (flush the boiler with fresh water again) and taste the water from the brew group. If you find it tastes slightly sour still, you need to flush again. If you want to be real nerdy, you can probably get a pH test and compare inlet and outlet pH of your water to determine how many fresh water flushes you need to do after descaling. It depends on how thoroughly you're able to drain each boiler, so no one can give you a hard number here, not even the manufacturer (even Synesso's descaling instructions don't specify an exact number, IIRC). Anyway, when the taste is gone and/or the outlet pH is the same as the inlet pH, you're done.
If you understand the details of what needs to happen, the job is half-done. The process is extremely intuitive and has a plenty big fudge factor on it. Even if you fail to completely get rid of all the citric acid (technically, you will if you only flush till you can't taste it anymore), the remaining solution will be far too weak to do anything bad.
I hope that's something like what you're looking for, though of course wholly unofficial.
*DISCLAIMER*: Doing stupid things with appliances that use water and electricity can result in death or serious injury. Always unplug an espresso machine before poking around with its internals, and DO NOT TOUCH hot espresso machine components. The above instructions are provided freely and AT YOUR OWN RISK.



