if you want an awesome arm band for the gt-31 you could get the aquapac but ive been using an iPhone arm band its a bit of a squeeze but works so well, and very waterproof because ive used it about 20 times with the gps in it and use the arm band for laps in the pool with my ipod in it and water proof headphones only prob i can see with it is that you can only fit one gps in the arm band
i payed $129.95 it costs a little more then the aquapac but looks better
h2oaudio.com/also available at any Ranger camping stores