If by task bar you mean the back, home, and recent apps buttons at the bottom of the screen (also called system bar or navigation bar), then their visibility is dependent on the app.
I think you are asking if you can make the buttons disappear when playing games in the same way they do when watching movies. Their disappearance when watching movies is also dependent on which media player app you are using, though I think most now automatically make the buttons disappear, or at least have them fade to small dots.
So the same is true for games. I just tried Zen Bound, but the buttons remained fully visible. Then I tried Osmos and Spirits, and they faded to small dots in each. There are likely games where they disappear altogether. Again, it's dependent on the app.
To force the system bar to disappear, and also to regain the screen space it used, you can install an app called Hide Bottom Bar. However, your device must be rooted. I don't use this, as I'm not rooted, but it generally gets pretty good reviews.
Of course, if you're rooted, I suppose you could install an alternate ROM that would have all sorts of customization options not available in Jelly Bean, including the ability to get rid of the system bar.