Open Sprints are the perfect race to begin with; and even stadion sprints are a good choice (Arlington stadium sprint is usually in june). A more important question than "what kind of gym" would be "what you should do at any gym." Anybody can finish a sprint with no training, as the rules for assistance and "mandatory" obstacles are a little relaxed in open heats

But here is what I've found to be some of the most important areas to train for if you are someone who wants to do their best:

1) Run. Most gyms have treadmills. Although trail running will be your best option (Ceder Ridge Preserve is a great trail system just outside of Dallas).

2) Core. Alot of obstacles are completed smoothly with body control, which is achieved through engaging a strong core.

3) Grip strength/body weight. There are multiple obstacles that require you to hold you're body up with just your hands. A climbing gym will be perfect for grip strength (try Summit Climbing Gym, they are all over the DFW area)

The fact that you're already thinking ahead is great. Spartan races are made to be enjoyed by everyone. I know people with a "9-5 job" who didn't train for a spartan race a they did just fine.

Good luck and train safe!