no problem... there's a lot of ways to shoot for a custom white balance.
The easiest way is to set your camera's manual settings for the light in the gym... using a light meter is probably the most accurate, then take a photo of something white, so that the white fills almost the entire frame. Then in the cameras menu settings, choose custom white balance and it will ask you to select the photo of the white object. Once you've selected it, change your white balance settings to custom.
That's all there is to it!
