In a semi you can have as many gears as you want and even aux. transmissions for more gears. Look at some of the HD trucks from the 50's and 60's and I think some had as many as 48 speeds! They were still diesel. I worked in a shop that built natural gas plants. They had to get a huge skid out of the shop. A hydraulic power pack was brought in to lift it so it could be put on a trailer. Once on the trailer, the hauler brought over an old REO(I think) to move the trailer to the yard pending delivery. They have a fleet of new trucks with a lot more HP but for the real big job brought the old one over and it moved the 100 ton+ skid by itself. There's a reason they aren't still using the IH 549 gas engines in semi's. Even with the most modern hi-tech gas engine, you wouldn't come close to the durability, fuel economy and efficiency of the diesel in heavy duty applications.
A Hemi is a decent engine but also one of the biggest frauds in the automotive world. Dodge owns the trademark and is using it to sell vehicles but that's all it is. The 426 and earlier Hemi engines were in fact true to the term Hemi. The new ones, not so much.