Problem is, it is copyrighted. All content, no matter who makes it, is copyrighted. From the World Intellectual Property Organization :"[Copyright] thus underlines the fact, recognized in most laws, that the author has certain specific rights in his creation which only he can exercise ... a created work is considered protected as soon as it exists, and a public register of copyright protected works is not necessary.."
Yes, this is a game mod, and the author of the mod does not exercise intellectual jurisdiction over the game, but to a certain extent the content of the mod is that author's original work. By not including the game mod link in your description, you are deny the mod maker recognition for their work.
This statement is not entirely correct. You are making money off unique gameplay, originating from a mod, which sets you apart from the many other YouTuber's making GDT videos. In order to properly credit all parties responsible for the creation of the unique game that you play (i.e. not a vanilla game, but a modded game which is different from other copies) you must credit both the developer of the original software and the developer of the mod which made your unique gameplay possible. Also, the components of the base game are not wholly developed by the developer of the game, not unlike modding, Greenheart used many libraries and tools in order to make GDT possible. Much could be learned from GDT's own credits page, which gives links to all others intellectual property.