There is a Gaggia-branded spray-on stainless steel cleaner a lot of Canadian espresso machine dealers seem to sell, for example here. I did a quick search on HB and over at CG and couldn't find any mention. Has anyone actually tried it?

Nope. I just use a microfiber cloth that has been very slightly moistened. I usually blast it with the steam wand. Then wipe the machine clean. No cleaners needed. Most of the stainless steel cleaners I have used leave streaky, oily residue behind. You could also use good old vinegar and water for those tough spots.