I've spent more time than I care to admit shooting in dingy gyms. Here are some tips:
1) Shoot at 2.8. That's why you spent the big bucks on that lens.
2) Shoot RAW files. You can do more to save an underexposed RAW file. You can also shoot at one color balance setting (right or not) then download the files into Lightroom, adjust one frame's WB to perfection then sync the rest to it
3) Fast glass like a 85/1.8 is helpful though I usually shoot them at f/2.
4) The best photogs will still get a lot of bad frames (with manual or autofocus) in dark gyms, so shoot lots of frames. Lots.
5) If you can afford it, upgrading the camera to a 5D Mark4 is worth it. The high ISO settings are very clean.
