Until GM offers a 10spd for the 6.6 gas, I will never consider one for my personal vehicle. I have two 2020 gasser work trucks for my business but my personal truck is a 2020 HC Duramax., The 6.6 gas is cheaper than the Duramax and gets the job done just fine but IMO they are not even close to being as pleasurable to drive as the Duramax. Long drives towing with the gas, lots of shifting and high revs whereas the Duramax quietly hums along with power to spare. Even if i didn't tow much I would prefer the diesel. The additional maintenance costs are well worth it! The 10spd in the Duramax is awesome, so don't forget about that benefit as well.
As far as the reliability, I have not had one issue with emissions on Duramax trucks and I've used them for plowing, short daily drives and everything in between. Don't let the comments about reliability and all the emissions issues scare you away, they are very few and far between. The Duramax/Allison also has a 100,000 mile power train warranty, gasser only 60k.
All that said, I would be very interested to drive the 6.6 gas with a 10spd transmission, I just can't understand why they wouldn't offer it. The GM HD gas motors are incredibly reliable, we have had some transmissions go on our gas work trucks but not one issue with the previous 6.0's and I'm assuming the 6.6's will be just as reliable. Drive them both and decide for yourself..