Fielding skill can’t truly be assessed inside a gym. Focus on their athleticism. Instead of hitting ground balls directly at them, have them start in the corners and hit the ball to test lateral movement. For the outfielders, work on drop steps and running down balls hit over their heads or cutting off long singles to the gaps. I have used tennis balls and no gloves for this drill for outfielders.
Catchers can work on footwork for throwing to the bases and “bad ball”.
Use the longest dimension of the gym to help base runners establish leads from first base. Combine with pitchers or catchers to work pick off moves or the catcher throwing behind the runner at first. It helps runners determine how far they can safely get off and it shows catchers how far off runners need to get before it’s worth throwing over.
There are tons of things that can be done in a gym.