It appears that for the 2018 model year the Mountain Master GT chassis is gone, at least it is not listed on the Spartan (http://www.spartanmotors.com/rv-range/)website. Looking at the current list it shows the following.
But with some more digging, it appears that what Spartan actually did was relabel their chassis models. Originally the Mountain Master GT (MMGT) was a 400-450 HP ISL based chassis while the K2 was a 500HP (ISM or ISX12) based chassis and the K3 was a 600HP ISX15 based chassis. That 500HP K2 had a beefier frame to handle the higher torque and HP that those engines produced. It also had a bigger radiator and I believe an Allison 4000 MH Transmission as well.
Now since the 500 ISM or ISX12 is no longer being produced, Spartan took the opportunity to drop one and rename the other. So now the MMGT is known as the K2. This "new to us" K2 is basically the old MMGT and is not the older beefier K2.
With the original K2 gone the K3 500 takes over the 500HP chassis slot using the newer 15 liter X series engine. This is the same engine "block" as the 600HP, but just tuned to the lower HP and torque ratings.