1) It has been always the case that the top SC 2 players were in Korea. This means, if someone could win 3 GSLs in a row, that person would undoubtedly be the best player in the world and such an accomplishment would be considered an absolutely historic one. And this was also one of the reasons GSL was so hype: all the best players are here and they are all trying very hard. This assumption has been ingrained in us for so long that we no longer think about it, we don't question it and we don't consider what it would mean if it were false.
2) However, Serral proved that this year the above assumption is false. He has dominated all the top Koreans except Maru. So we can no longer assume that "all of the contenders for the best player were in Korea". As a result, we should look differently at those accomplishments by Maru; this year's GSL did not involve all the best players. I don't think anyone can dispute this fact, unless if you think Serral is not among the top 4 best players in the world which is a ridiculous assumption. So even if you place Serral as the no. 4 (which again, I cannot see how you can place him lower than no 2), the logical conclusion is that the GSLs this year did not involve all the contenders of the best player in the world. But of course, the WCS tournaments also did not involve all the contenders and in fact objectively, WCS were far less densely populated with the "contenders for the best player in the world" compared to GSL.
3) So what does this leave us with? I think with the reasonable conclusion that Maru and Serral are both no. 1 and no. 2 but so far Serral has had a better showing in the tournaments that they both have competed in this year with the caveat that they all have been "weekend" tournaments. On the other hand, the GSL format is also a little bit weird with the player selected groups so all in all, we do not have a definitive answer to the Serral vs Maru question.
Could Maru beat Serral if Serral were competing in GSL? Absolutely, but the other way around was also absolutely possible.