Gnomes aren't exactly a race you put into the vanguard when sieging a fortress. They are those that build the warmachines, like those siege towers.
There was a gnome in the classic server teaser
Long standing joke/fact that no gnome ever made it to a cinematic.
Nelf was in classic, belf in BC, orcs are in pretty every cinematic much but them gnomes...
They do not die; they do not live. They are outside the cycle.
I was a bit surprised that they used night elves of all races as the only mages in that cinematic. It would have been a good opportunity to show a gnome I think. But I guess they must reuse assets somehow.