New a this hobby
Assuming the average level of light in a gym is quite gloomy (as far as photography is concerned), there will be no perfect solution and, depending upon just how far away you are it would be difficult to give a precise answer.
That said, the combination of lens and camera you have is likely as good as you will get, unless you get into the professinal grade.
The bottom line is, if you need greater reach (zoom) and faster focus, the price will begin to rocket. I would have thought that the combination you've got would do pretty well, though, even shooting wide open at f2.8.
If there is enough light you could always try a 1.4x or 2x teleconverter first to get you closer, although there is often debate over whether simply cropping gives you as much detail as opposed to the degradation due to the teleconverters.
2013 : My new year's resolution -
To not post anything negative. If I don't have positive advice or support, even if I'm wrong. I won't say anything at all!