Permission needed from the author and the publisher of the guide.

contact support via:

https://support.gog.com/hc/en-us/requests/new

and tell them all information. They might be able to sort something out. And if you want to get extra points you could contact:

http://crownpublishing.com/contact-us/
https://permissions.penguinrandomhouse.com/

(According to their own list prima publishing is handled by penguin)

the publisher and try to get their approval prior of contacting GOG ;)

Since permission has to go through the publisher, anyways, I imagine that they could either get an already-made PDF of the guide or make a better-looking PDF than the one from your likely-beaten-up copy. In other words, I don't think that your physical copy would be useful and you're probably better off saving yourself some trouble and hanging onto it (surely it has some sentimental value to you, after all).

You certainly don't want to end up in the situation that it looks like that Jagged Alliance fan got into in which he sent them his guide and, two years later, a digital copy still isn't available with the game and the physical copy was never returned to him. It's reasonable to contact them and encourage them to look into adding the guide, but I wouldn't give up your copy with any expectation of getting it back or seeing it available with the game.