I would say that Gearman is better for queuing “jobs” and RabbitMQ is better for queuing “data”. Of course, they are both really the same thing, but the way it works out for me is that if you are trying to “fan out” work to be done, and the workers can work independently, Gearman is the better way to do it. But if you are trying to feed data from a lot of sources down into fewer data consumers, RabbitMQ is the better solution.
The history of RabbitMQ, as something that allowed Twitter to take bursty loads of messages, and feed them into crusty old SMS gateways that could keep only one connection open, were rate limited, and didnt have retries, is illustrative of the kind of problems that RabbitMQ is good at solving.