For a 9000 lb. trailer, a gas truck will tow it just fine. It will on occasion shift to 2nd and 3rd doing so and run 4-5000 rpm. It will hold speed on 99%+ of the miles you drive as good as a diesel truck. There are hills out there that it will be a few mph slower on.
Gas is likely to result in a slightly lower total cost of ownership for most than a diesel.
If you aren't afraid of the sound of an engine revving and want to save 10k up front get a gas truck. If you can't get past the feeling the truck is hurting or struggling when it is north of 4k rpm for a few minutes, or if you just plain want a diesel then get a diesel. The diesel may have a slightly lower cost of fuel which eventually will offset the up front price and most diesel truck will get some of the cost of the diesel engine option back at resale time.
If you think you might want a trailer over 14k lbs eventually then the diesel starts to make more sense for more people. Not that the gas engine won't get it there at that weight or heavier but it is getting to the point where the truck will have to downshift to 1st on some of the steepest hills.
I personally own a gas truck and tow around 10k lbs and am perfectly happy with it.