Gautam Gambhir's position becomes even more indefensible, after he claimed that the elbow wasn't deliberate during his press conference. If it wasn't deliberate, then why pleady guilty? Gambhir's history in this matter doesn't help either. His previous encounter with a match referee was for a similar situation involving Shahid Afridi in 2007. The harsher penalty second time around is most probably a result of the fact that this is a repeat offense, and it is hard to see why it is not deserved.
Gambhir clearly revels in getting involved with the opposition, and it doesn't seem to affect his concentration. But he needs to make sure he avoids this kind of gratuitous contact. As a batsman, he is always going to be on the wrong side of the law in these situations.
In this instance, Gambhir had no business running into Watson during his second run. As a batsman he's obliged to run around the bowler in these instances.
Update: The BCCI has indicated that Gambhir will appeal. This is probably an attempt to get the ban converted into a hefty fine. There isn't a good reason for this to happen, especially since Gambhir pleaded guilty. I don't think a guilty plea guarantees any sort of leniency.