Searching Apple Maps for "Gary Glitter" leads to a local map of public toilets. As posted to Hacker News by Robert Trencheny, it worked for me with in-app searches and for the URL
It's Cockney rhyming slang for "shitter", though whoever worked on this easter egg might not be aware that the connotation is mostly anatomical rather than architectural, and thereby deepens holesome implications not in keeping with Apple's wholesome image. And then there's the matter of Glitter being one of Britain's most famous child sex abusers, imminently to be released from prison…
UPDATE: Apple has removed this feature.