Aperture vs ISO for action shots

You can buy a standalone incident light meter. Go out onto the floor and point towards your seat or shooting location. The meter will give you a set of aperture and shutter speeds that can be used at a given ISO setting. In general the exposure settings are a Log base 2 series of numbers, so you can mathematically calculate what set of three numbers are equivalent. Usually this is call the EV or exposure value. In most cases with modern camera you really do not worry about the EV values. what you need to learn is the limits of the camera in terms of what you are shooting.

Minimum shutter speed you can handhold for still images, generally 1/(2 x focal length) seconds. Can be slower if using IS

Minimum shutter speed for action, depends on the action but for sports 1/250 or 1/500 would be reasonable.

Maximum and minimum apertures. Most of the time in sports you are shooting at the maximum aperture, but in other situations you would use smaller apertures to increase the depth of field. Modern crop cameras also have issues with diffraction effects once the aperture gets smaller than f/11 to f/16

Max ISO. this is very camera and viewer dependant. For a modern DSLR crop body ISO 1600 should be OK, unless the image is being really printed large.

Some cameras have a auto ISO manual aperture manual shutter speed mode that works well for sports. Set the aperture as large as you got, set the shutter speed as fast as you need, then let the ISO vary the exposure.

If you are using a 2X tel-extender, except for some that are matched for specific lenses you will have to stop down from the maximum aperture to get an OK image. Playing around with non-matched 2X extenders I have generally found I get as good an image just magnifying the image from the original lens 2X compared to using the extender, at least until I am using small apertures.

If the gym is uniformly illuminated many shooters use manual mode, setting the shutter and aperture as described above then adjusting the ISO until they get a good image. Then just leave it locked in for the rest of the shoot. While you doing that you can also do a custom white balance.