Gas or diesel

3500 for sure good argument for a dually.
Diesel for sure if you want to travel more than a few short trips a year.
15500 is heavy and if you plan to drive around in the mountains you will love the diesel. If you plan to full time you will love the diesel. Because we now almost exlusively use our TC we switched from our 2011 3500 duramax to a 3500 6.0 gas. There is a huge difference in the two pulling a heavy trailer.
In my opinion a 2500 is out of the question.
Also in my opinion ask all the questions you would like. If people don't want to comment they can just go to the next post.
JMHO

You didn't ask my opinion but that trailer is too much for a 2500 with 3,000 lb on the pin. Go 3500 dually or a lighter trailer.
Now to your question. With that size trailer you will be a lot happier with the torque and fuel economy of the diesel. Maintenance is more expensive but intervals are fewer so that's a wash. Gas is cheaper up front and is fine if the truck is also to be used as a daily driver but fuel stops will be more frequent when towing. After towing with diesel I will never go back.

2500 is too close for 15,500 IMO. Get a 3500.

Are you going fulltime or just parttime. IF fulltime I would say get a 3500 and the diesel. If you just plan on a trip west now and then get the gasser I would still go with a 3500.