If you shoot in jpeg then set a custom white balance by shooting a sheet of white paper at the start of the event under the gym lighting. Make sure that having set the custom WB you actually use it to shoot the photos. That will get things pretty close for all subsequent photos. If you have a proper WB card then so much the better, but I think a sheet of white A4, or even a white shirt will get you in the right ballpark.
If ou shoot raw then you can do exactly the same as above and/or you can fix or tweak WB further in your raw editor, with no loss of image quality. Highlight all photos at once and use "click" white balance to set the WB for all photos. You can use that sheet of white paper that you first shot, or a white shirt from any of the photos shot during the event. If you want to tweak further, for artistic effect, then you can easily nudge the colour temperature to taste.
Personally, I would shoot raw, with a daylight, or uni-WB setting, and then sort the WB in my raw editor with a click on something white or grey and a nudge to the temperature if I wanted to change the ambience a little.