The existentialism im familiar with is more a part of the 3u course.
I think that existentialism is all about choice, in a world that lacks absolute meaning.
As in, the only meaning you can get is what you make for yourself and in discovering yourself.
In R&G, they are fated because they are *not* choosing. The point is that they should not merely accept their fate. It's hard to analyse r&g wholly within existentialism, considering all the other influences, so you can twist the ideas a little.
anyway, i've come to the conclusion that the existentialism they teach in School and the existentialism you would learn elsewhere is different.