I came across this phrase in “A Terrível Criatura Sanguinária“, a short story by Nuno Markl, which I read at Hallowe’en. Yes, I’m old enough to use an apostrophe in Hallowe’en.
Literally, it means “wet cat” or maybe more like “dripping cat” I think – the “pingado” is related to “pingo” in Pingo Doce, a chain of supermarkets. I had to ask because gTranslate was utterly baffled.
Idiomatically, apparently, a wet cat is someone at a poorly attended event, or who maybe was paid to show up. As with a lot of things, the exact meaning varies with time and place. In the story the protagonist worries that only a few Gatos Pingados (stragglers just there for the free food perhaps?) would shown up at his funeral and it’s been shown that there were a few Gatos Pingados (paid supporters who hire themselves out to pad out an audience) at Trump rallies, for example.