Regarding system-specific stuff, that isn't a big deal. Game systems -- the rules by which they work -- are not copyrightable. They're patentable, but it's pretty rare to even hear of anyone trying to do that (I think there's something associated with MTG), and there are very strict limits on how and when you can go about getting one, and for 3d ed. GURPS, it's a non-issue.
Now, you couldn't just copy the written expression of the rules, but so long as you explained any concepts you needed to in your own language, it'd be fine. This last point is just to avoid running into the copyright that SJG has on the way that they described the uncopyrightable rules. Trademark issues likewise seem minimal -- they have to tarnish or cause confusion among customers, and I doubt that anyone is going to think that a mention of hit points will confuse anyone.
This is why I find the whole d20 thing so bizarre. I'd bet that a lot of the stuff published under it could have been regardless. All you get is the name and the mark, and even then you could use the name unauthorizedly in a nominative use sense. That is, not to claim association with, but merely compatibility with. Some circumspection is called for, but it's not bad.
Derivatives of something else -- what you're calling licensed properties -- those you would have a hell of a lot of trouble with OTOH.
Now, you couldn't just copy the written expression of the rules, but so long as you explained any concepts you needed to in your own language, it'd be fine. This last point is just to avoid running into the copyright that SJG has on the way that they described the uncopyrightable rules. Trademark issues likewise seem minimal -- they have to tarnish or cause confusion among customers, and I doubt that anyone is going to think that a mention of hit points will confuse anyone.
This is why I find the whole d20 thing so bizarre. I'd bet that a lot of the stuff published under it could have been regardless. All you get is the name and the mark, and even then you could use the name unauthorizedly in a nominative use sense. That is, not to claim association with, but merely compatibility with. Some circumspection is called for, but it's not bad.
Derivatives of something else -- what you're calling licensed properties -- those you would have a hell of a lot of trouble with OTOH.