Using something someone else made is highly lame though.
That's like the Stargate and Starwars stuff. Everyone has to copy partly, of course, because there's nothing original in the world that hasn't already been done. But to copy name, power, and such? That's going overboard. "The Force," Anyone reading that would know Starwars right away. Unless it was just, the word, "Force," used differently.
"Atlantis, Ancients, blah." Those used alone are not copying, but to use them together, anyone would think Stargate.
G-Man alone is not copying, but using the same power, and apperance. Anyone would know Half-Life automatically. Name it something else, change the apperance, power, anything. But don't let people automatically think Half-Life or it's full out copying.