For the last two years my clear water tank (about 8 gal.) has been mostly
empty and unused because I was too lazy to look for and replace a bad galley
pump.
I am now the proud owner of a new working galley pump.

I have already coughed-up the $$ for a gallon of Simple Green, so instead of
shelling-out even more bux for the expensive, "official" Level Gauge
Cleaner<tm>, would using Simple Green (sloshed-around by towing) be a good
cleaner for my gray tank?  I suspect it would.