What about the stuff they use to descale dish washers?
I had to look it up
The prochem stuff is sulfamic acid ... you can get it from mistral etc also but its not expensive from prochem so probable not worth sourcing it ... although who knows what concentration your getting from prochem .
Many of the Coffee machine descalers are the same stuff ... they wont wnat to damage the Gaggia after all ..
The kettle stuff ? ... it might be mostly citric or lactic or a combination of acids .
The sulfamic might be chosen for being less harsh on certain metals perhaps ... although is a HE stainless steel , figured u could throw anything at that ?