I have stronhold in my GOG library and browsing steam I found the same game and noticed that it is in dlc but that same dlc is not in GOG, I want to know if it is already included in the game in GOG or it is not found here.

It's not available on GOG as a DLC, extra or goodie, or present in the game. I had never heard of it. Apparently all it does is changing the campaign cinematics map to make it look like you're conquering the USA instead of England (no gameplay changes).