In a way, this reminds me of the John Terry - Alexis Sánchez incident from Chelsea's Champions League semifinal second against Barcelona. Except, as far as we know, Terry's innocuous little howdy-doody with his knee into the lower back of Alexis was completely unprovoked. Alexis crumpled to the ground*, Terry was sent off, and Chelsea won the European Cup. So that turned out alright. Unfortunately for Arsenal, they were not so handsomely rewarded for a petulant action of one of their defenders.
* I like how Costa just went to complain rather than feign injury. Work the system directly.
Costa and Gabriel have had priors, as the incident from Villareal vs. Atlético Madrid from a few years ago shows, and if lip-readers in Brazil are to be believed, Gabriel is certainly no fan of Costa's switch of allegiance to Spain, either. But most of what happened Saturday was probably simply down to the Chelsea provocateur successfully riling up the younger man, starting with Gabriel jumping in for a bit of afters from the Koscielny incident, continuing through the matching yellows and constant back-'n'-forth chirping, and ending with the little flick of the heel, which Gabriel tried to disguise, hilariously, with a hand motion as he were some low-rent magician. (He made a huge mistake!)
While there are rumors that The FA will look into the Costa - Koscielny incident for any possible retrospective action, the Arsenal match was another example of Diego's ridiculous ability to toe the line and get others in trouble while avoiding a similar fate for himself. And for that, we love him.