It's more of a mix with the books rather than the show the models obviously based off the show. It's got some elements of the show but not many.
Well, that's kind of an understatement, almost the characters are from the show and most of the story is more or less based on GoT. However, it's not not really neccessary to have seen the show to understand what's going on.
In a way, it adds to the atmosphere, of course, but then again, it's clear that the game is going to explore the story in a different manner, so it's totally ok not to have any preset expectations.
Having finished the current build, my main worry is that I can't see how, at the current development pace, the story is EVER going to be brought to a satisfying conclusion. Having dabbled in DAZ and RenPy a bit myself, I know it isn't easy, so don't take it as a sideswipe against the devs, but one has to be realistic.
Honestly, I feel the game deserves a ton more patreons, as what is there is definately better than many other games with much bigger numbers of supporters, AND you have the GoT appeal on top, but that leaves the devs in a bit of a catch 22, not enough income is generated to warrant a much higher dedication to the project, but without a higher dedication, it'll be tough to get more supporters and thus income.