Yes you can. Anyone can redeem the gift card - it doesn't matter that it was given to you.
I'll be selling mine in the next few days. I already have around £160 in credit so I really dont need any extra credit!! I had £180 last year, so haven't used much! Will spend a little on Mavericks, but to be honest I already have all the apps I need for now.
Maybe this fall? There was an article on MacRumors earlier today, I suggest you go take a look at it.
It didn't mention iWork for OS X being free but it seems iWork and iLife for iOS will be free which would only make sense for OS X to also be free too. Plus Apple is doing all the iWork in the Cloud thing.
That's a bit of a stretch. iWork has limited uses (just like all productivity office software) on a tablet. That's why it will be free because it is limited. I doubt (but could be wrong) that iWork for OSX will be free since that's where most of the production will be. On top of that, it is also much more powerful than the stripped down version in iOS. I'm not saying it isn't possible, but generally I only use iOS versions of software to display what I have done previously in my desktop versions. Rarely do I create with my iPad. As a presentation tool my iPad rules, but not as a production tool.
Edit: And as pointed out already, this is in the wrong forum.