@DarkMahesvara >they do not own the modifications made by somebody else
A government monopoly can only be held - not owned, no matter what the confidence tricksters known as lawyers say.

In this case, such modifications weren't originally permitted, thus the author didn't gain copyright restrictions.

A copyright holder can later decide to authorize previously illegal modifications under terms that suit them (even if it's not in the author's interest) and rockstar did just that.

>it depends on the country but you can own the rights to derivative work.
In most countries, the original copyright holder can authorize that derivative works can be under different terms.