Twins almost.
You should be able to call FSI or Simuflight for some advance study material. I used Simuflite and they were very accomodating. As far as similarities, they are almost identical unless you are flying an AC GIII, the rest of the systems are practically the same. There are some obvious differences in limitations, weights, avionics ( almost all are a little differntly outfitted anyway), and the AC GIII has first generation glass. Easy to use too. If you were able to get your hands on a GII manual and study engines, hydraulics, fuel, flight controls ( again minor differences such as the ground spoilers), landing gear, brakes, fire detection & protection, and a few other areas, you will be ahead of the game some.
You will love the Gulfstream, my favorite plane to date ! I flew the DC GIII, the GII without winglets and one with, all great planes ! Sure the Aviation Partners winglets make a huge improvement in looks and they give you another 200 NM range. I flew the GIII first and my first GII before actually going to GII recurrent, the G1159 covers the GII, GIIT (tiptank), GIIB, and GIII AC & DC aircraft. If you have flown a lot of different Lears then you will know what I mean by differnces, radios, FMS, and other pilots prefferences.
You will most likely have some GII pilots in some of your classes as they do at Simuflight during system training.
Nothing like 11,400 lbs of thrust a side !!!! Wait till you fly it light !
Have fun and congrats on the upgrade !