They support the major online stores and simply and kindly asked people jumped buy it on GOG. This is just a reward for doing that. I actually think a comic is pretty cool. Definitely unique and at least it's not an exclusive dlc
That said this will be all over the internet within a day of launch. So not a big deal.
I pre-ordered ages ago, and tbh I've skipped most info about it, so not overly hyped, I like to go in knowing as little as possible it worked for all 3 witcher for me