I grew up less than a half mile from the Gila river in small town New Mexico. My parents still live there. I'm going to do a trip there in the spring, hopefully. I spent most of my childhood and adolescent years floating, fishing, and swimming this river. The Gila is a beautiful river but can be very remote in some areas (the wilderness section and the middle box). It would be a good idea to have somebody who has done these runs before. Anything between 250 cfs and 1,000 would be good. Watch out for strainers, root clumps, fences, irrigation diversions, etc. Also, check into public and private land ownership on the middle box.
The major obstacle on the Gila is the flow. The flow is very inconsistent/unpredictable and depends on recent precipitation (usually rain instead of snow). Some years provide good snow pack resulting in a more normal runoff in March and April. If there is little snow or a drought year, you basically need to time a heavy rainstorm that could occur anytime. With this latest storm, the Gila ran at sevral thousand cfs, but has already dropped to appox. 600 cfs today. It happens quickly and is very hard to time a trip. It was much easier when I lived there.
Be very careful with flash flooding. I have seen this river run as high as 14,000 cfs. A couple years ago it peaked in a January rainstorm (on top of snow) at 20,000 cfs. There were lots of cottonwoodd trees, dead farm animals, and all other types of debris floating down river. A good rainstorm in the mountains could mean the difference of sevral thousand cfs. Check the weather and be careful.
The middle box has more difficult water but is only 18 miles long. The wilderness section is 30 some miles in length and has less whitewater, but is super remote. If you have an accident in there, your group could be facing an epic hike and or rescue.
Overall, its a tough trip to make happen if you're not a local. Although, it's a great river with beautiful scenary.
Hope this helps,