I might be wrong, but it was my understanding that Gretsch wooed a lot of the top name '50s-60s jazz guys to play and endorse their kits by offering them a free drum set of their choosing if they'd agree to allow their name and likeness be used for advertising. Instant brand recognition with the hippest drummers of the day.I also heard that no money ever exchanged hands - just the free drum set. AND, if they wanted a new set, they had to return the old one first.
Gretsch will tell you that they never provided "free" drums. Their ads from the era quote the players saying they are the best drums they have ever owned.