There is a good possibility that we may initially start to see some appear into dealers by May of this year. Built in Toledo Ohio, the Gladiator will have a great loyalty following. Jeep's plan is to have over 200 parts & accessories ready to roll also in the second quarter of 2019, which is really good news as well. Some of the accessories will include, 35-inch tires, LED lights, tube doors, bikini top, and storage drawers that pull out of the back. Uconnect infotainment/ Apple CarPlay and Android Auto compatible, and touchscreens as large as 8.4 inches will be available. MotorTrend's first look provides a good insight & I hope you will enjoy this article! There is a very good gallery of images located here as well. https://www.motortrend.com/news/2020-jeep-gladiator-first-look-review/
I think 35" tires will be an option, or at least available as a dealer installed option, and should fit with no modification/lift. Above a 33" tire though, I think you start running out of "Truck" tire options and would be more of an off-road type tire. Not sure how well a 35"+ mud terrain would hold up to being used as an actual truck/towing tire??