Fatguy24 raises this valid point to this post:
I don't see why the Yankees can't void his contract based on his testimony in the Balco case. Court cases are a matter of public record and the testimony he submitted before the committee should be used against him.I'm not a legal expert, but even though there supposedly is some steroid-ish clause in Giambi's contract, there was technically no steroid element in the collective bargaining agreement. To prosecute, even within the baseball world, would be a bit of an ex post facto move.
I would also lean toward the conclusion that the grand-jury hearing is not necessarily public record in the true sense because his testimony was sealed, then leaked. Without the original documents in hand, the Yankees would have a tough case, and before an arbitrator, it's unlikely he (or she) would rule for them based on a leak. Also, perhaps Giambi has federal immunity. While that wouldn't protect him from baseball, per se, (see: Shoeless Joe Jackson), it couldn't hurt his argument against those who would suspend or void him.
Now, could the commissioner suspend him for conduct detrimental to the game? Probably, and if it wasn't too severe, he could do so without a big fight from the union. If it were a couple of years ago, Giambi may have even negotiated his own punishment. But now seems late in the game for that, as well.
As far as I can tell, though, the using-his-testimony has its loopholes, and by the time any legal avenues were worked out, he'd probably be through his contract, and it's unlikely the Yankees could recoup any salary paid during the legal struggle without going to a civil court. And I don't like the chances of them winning there.
If someone has a better handle on this and knows I'm way off-base, let me know. I'll be glad to correct the record.

I can't argue with any of the legality of what you said but the immunity he received the sealed testimony that he gave should only protect him from any legal matters. Baseball is a business and lets face it, they don't play by the same rules as the rest of society has to.
For instance, remember when you filled out your job application and they asked if you have ever been convicted of a crime? The law says your juvenile record is sealed but a company can fire you if you don't put that you were convicted on that application. I am not sure if its legal or if its right but companies have done it before (maybe under the guise of another reason) and will do it again.
Honestly though, I just want Giambi gone. Never really liked the guy to begin with. Yeah, he did amazing things in Oakland but thats Oakland. He is like Mike Sweeney. Sweeney was great as a Royal but could you be sure if he would be great on a serious contender? Giambi was overrated from the get go. Who brought him by the way? Oh yeah, the guy who's job is in jeporady at the moment :)