Given that the GYMBOSS is not water resistant, this seems like a must.
It's thin, and fits very closely over the timer. You'll need to work it around a bit to get it seated just right, but once you do, it fits quite well, and the gap for the belt clip is no bigger than it absolutely needs to be.
Because this is so thin however, it won't give you any protection against drops and shocks - this is strictly to prevent damage to the device from sweat, rain, or splashes. For that, it's just the thing.