Maybe a backup plan in case Valve didn't approve of the Portal Gun.
No. That would be silly, like copyrighting platforming. For a better example, while you can trademark a brand, it's impossible to patent a recipie. So while you can't mix up chocolate exactly the same as a Snickers bar and sell it as a Snickers, you can sell it under your own name (provided you actually made it, not just ripped the wrapper off a Snickers!).Counter to that, then wouldn't the concept also be off-limits?
I am not trying to be a pain, I just want a solid answer with little reason to be able to fail.