Will the programmer be subjected to torment in the grave if others use his open-source game engine to create games that contain forbidden elements, and he is unable to control their uses due to its GNU General Public License? And what is the ruling if the codes he writes are not directly related to forbidden things, such as image generators and water refraction codes?

If the predominant use of this engine and open-source is for permissible games, then your work is permissible. However, if its predominant use is for unlawful purposes, then it is not permissible. The rule is to consider the predominant use of something that can be used for both lawful and unlawful purposes. If the predominant use is unlawful, then it is not permissible to assist in it, due to the prohibition of cooperating in sin and aggression. It is stated in "Fatawa of the Permanent Committee" that anything used in an unlawful manner, or predominantly thought to be so, is forbidden to manufacture, import, sell, or promote.