He didnt elbow someone, he brushed him as he ran past, of course if Watson wasnt being an idiot (Ok, probably asking too much there) then it never would have happened.
Bowlers and batsman bump into each other quite often and usually more forceful than that, you dont see them getting charged
Gambhir probably copped a fine but thats all he deserves, it was stupid but Watson was in his way, leaning into him running his mouth whilst he was trying to run and impeding his path, Watson was at fault fully in the incident