Will the GoG version include all the DLC characters?

Yes... it is confirmed... many LP-ers have found various dlc characters in the game already...

I've unlocked one out of a coffin during a level, so one would have to assume they are littered throughout the game.
I received the female explorer in a yellow suit.
If that is a DLC character (I haven't checked), then it is confirmed.

According to this:
http://spelunky.wikia.com/wiki/Spelunkers
The "Round girl" is a DLC character.I found her,so I can pretty much confirm that DLC characters are in game.