I've done a sliced loaf style several times on the pellet pooper. I have a link to the recipe that I loosely based it on, just have to find it. I've done a butterflied leg of lamb grilled also. Shawarma means a lot of things to a lot of people depending on location. Spent a few years in the Navy... had a few in a few different spots.
Edit.. here's the link. Gyro
. I made a bunch of changes, but the idea is still the same. I used a cast iron loaf pan among other things, but the seasonings were a bit off. Oregano, Marjoram and Thyme can easily overpower so second time around I halved everything. Took a few tries to make it, but that's the way it goes.