The last time that I checked and purged my spam folder in Gmail was before Christmas. Since then Gmail spam filters have picked up over 500 items received, with no false positives.
During that period of time, about a dozen spam messages have slipped through Gmail spam filters. That's less than 0.025 of the total caught. They weren't caught by Apple Mail filters, either. I've marked them in Mail and reported them in Gmail.
In Gmail, among the messages flagged as spam were a couple that caught my eye, and I opened them to to take a closer look. Indeed they were spam, and preceding the body of the messages Gmail had inserted a readily visible warning: "This message may not be from whom it claims to be. Beware of following any links in it or of providing the sender with any personal information." Moreover, Gmail blocked loading of images embedded in the messages.
Not bad, eh?