Vigilance
Explorer
That's comparing apples and oranges; except for D&D, and some FGU RPGs that are eternally in print, no setting-free games survive from the early era of RPGs. The elements that 21st century editions of Gamma World copy from the original and the same elements that GURPS Old West "copied" from Boot Hill.
And yet, people want to make games called "Gamma World" and "Metamorphosis Alpha", and not games called "Boot Hill".
Or put another way, the name Gamma World still carries weight after 30 years. That's why it continues to exist as a brand. The name Boot Hill does not, and so it doesn't.
In fact, companies have PAID for the right to call their game Gamma World, or to call their computer game Temple of Elemental Evil but not the others I mentioned.
And just because Top Secret, for example, didn't have a setting, doesn't mean it couldn't have been used for branding purposes. If AEG thought the Top Secret name on their RPG would have moved more units than creating a new brand (Spycraft), they could have paid Wizards for the right to call their d20 spy game Top Secret, just as White Wolf paid for the Gamma World name.
That seems pretty apples to apples to me.