I'm going to answer your specific concern here. As background, I have owned Duramax trucks for years, my most recent was a 2020 3500 with several versions prior to that, I think they're great. My most recent truck was my first gas in a LONG time, 2024 3500 HD HC with, as you know has the 10spd now and I'm really getting to like this truck!
First, If I were you, I would put more weight on feedback from owners who have the 2024 Gas/10spd unless you plan to purchase used or a 2023. There is nothing wrong with the 6spd but the 10 will provide a different towing experience. My feedback is based on the assumption youre buying a 2024.
Now, per your question, your main concern is "Duramax having emissions issues" or "towing with max capacity with gasser".
First I'll address my experience with "Emissions Issues". If that is your concern my experience is they are few and far between, I've had excellent luck with the L5P. Are you going to read stories on the internet forums of disasters, yes but are they really that wide spread? I'd say no. So if that is the concern, I'd say rest at ease buying the diesel. By the way, I short commmuted mine for years and used them for plowing with frequent low RPMs with no issues at all.
Second, and I'm going to use your numbers and the side by side comparison of my gas HD vs the Diesel. "towing at max capacity with gasser". You get a slightly higher 5er/gooseneck rating with the diesel but only slightly. You're giving yourself less then 350lbs to spare on rated PIN weight so getting very close, and that is assuming empty weight w/o all your gear water etc..Now if you meant maxing out on power, you won't be. Will it tow like the diesel? No, not even close but it will get the job done for you, just need to be a bit more patient.
My opinion givern your trailer...either by a Dually given the weight of your trailer (and IMO get the diesel then) OR buy a smaller trailer then go with the gas.