It says before you start the game you get a reward for beating it on hardcoe what is the reward?

An achievement/trophy.

You unlock an achievement for NV on Steam. That's it.

Oh that is dumb I was hoping for like a gun

Some of us just like feeling like completionists...my other friends are blown away that I'm 48/50 of achievements right now. Some people think it's garbage since it has no value, but for many of us it's proof we're scraping every cent of value out of the game and owning it. And hardcoe gives you 100 Pts which is the highest in the game.
It's also a way to monitor your completion of the game (you may realize that there are more than three ways to finish the game or forcing yourself to play in a different manner or toon set.)
I have nearly 18,000 gamer points which gives me some pride in my devotion to games.

I was thinking exactly what you were thinking, but no gun just a acheivement/thropy. or valve award.