Since there is no deluxe edition like on the Steam platform, is it safe to assume this version available on GOG includes the deluxe edition content? The store page does not state whether it has the content neither, and there is not a dlc for it. If it does not, is there plans to include it?

Nope, the deluxe content isn't included. The only version on GOG at present day is the standard version of the game. Maybe. They sometimes add DLC at a later date for games, and they did that with some of the Neppy games before, so it's possible it will appear on GOG. But as to when, nobody could tell you. If you really want the deluxe content, then it would be best to wait, because you won't know how they'll release the DLC. As in, they might release it as a whole package and not separately, so if you buy the game today, you'd have to buy it again to get the deluxe content. They also might give a steep discount to existing owners to buy the deluxe if they do that, but you never really know what they might do.

So yeah, if you really, really, really want the deluxe stuff and you want to save money, then it would be best to wait. But the catch is you don't know how long you'd have to wait, it could be a very, very, very long wait time for the deluxe content.