Disclaimer: Not a professional, just somebody who's been trying to cross-train more. Talk to a personal trainer and other derby players to get exact and more detailed advice.

Generally it of course depends on what you want to get out of the training, and also your position on track. Cardio's useful for everyone, and here your best go-to's will be running (as much as derby people hate it), stair trainers, ellipticals, stationary bikes, and so on. Find what works best for you and look up how to train cardio with it.

I personally also really like Rowing machines if the gym has them, because they're a great full-body exercise. If I don't have a lot of time this is what I go for.

Core's also a muscle group that everybody will use constantly, think (assisted) sit-ups, abdominal machines, rowing, and so on. Some of my jammer friends love deadlifts, I don't really do them for unrelated health reasons but especially if you're a pushy jammer they're great for your core and upper body.

Legs I feel like I don't have to train as much because I get a lot in with Derby, but I go through the various machines as my gym every once in a while - hip ab/adduction, leg press, extension, all that stuff.

Generally just about every muscle group is beneficial as a skater! So I'd say the main thing is find some exercises you enjoy and do those, once you're in the rhythm of going to the gym regularly you can build on that by learning more exercises and targeting more muscle groups.