G.W. E 100
GW E100 is really a made up vehicle. This has been confirmed by WG developers.
Also if it was a real Geschützwagen, armor would be very thin armor plate and thicker carbon steel plate like that of the Grille/Hummel/Nahsorn/GW Tiger. I am not sure how to deal with the History section of this page. Also there are very real T10 alternatives(several in total but only 2 good/useable designs) that were either drawn up or prototyped or produced in Germany during WW2.