It all depends.
You should add how much FPS you're currently getting and other info such as budget and your plans for the computer. Is it only for playing gunz?
In my opinion, I wouldn't upgrade the video card solely for GunZ but rather the long term. Black friday just passed sadly, but if you save up ~500-600 dollars max you could get a solid computer that'll last you a few years. Also, what FPS do you want?
We'll see what others say, but I can't really see a mild video card upgrade adding too much, and I wouldn't advise it just for GunZ alone. There have been many many great players who played with <100 fps though I could see how it helps especially because players are moving quite fast these days.
By all means however, feel free to upgrade your video card for probably less than 80 bucks if you wish.