Confused by this as well. I appear to own Arma II, but not the other Arma II. Should I enter?

And once you click the links on the bundle, they lead to .... the same page (https://store.steampowered.com/app/33900/Arma_2/). And it doesn't register my ownership. This is so, so weird

Not sure what's up with it either, it's a weird page.

So now I am wondering whether what is being gifted here actually matches the selected package - i.e. both "versions" of ARMA II or if it's one or the other. I doubt there's a way to know prior to redeeming the key.