I believe the vehicle/manufacturing CARB/EPA rules/regulations make it much harder to produce an aftermarket replacement primary gasoline tank compared to diesel requiring a lot more certifications/testing. That's why you see lots of aftermarket options for diesel, but basically nothing for gas unless it's an auxiliary tank. No idea why, perhaps it's because gasoline is more flammable/explosive etc. You can of course use external auxiliary tanks to transfer gasoline from into your vehicle, they just can't directly feed the vehicle, again no idea why they differentiate, but they do. I think Transfer Flow just recently did a 58 gal gas tank for some of the big Ford Super Duty Gas pickups, evidently the first of it's kind to be approved, but it took almost 3 years, and I believe the cost of just the tank is pushing $3k. Maybe it paves the way for more options, but at $3k how many 1/2 ton guys are going to drop that for 30-34 gallon option. That's probably my entire fuel cost for a year, but if it's your work truck or you do a ton of towing, overlanding I could see where it might be worth it. GM should just pull their heads out and offer a larger tank option.
There's plenty of room under the truck for a larger tank, 30+gal should be easy. The room from the Def tank alone is over 5 gallons.