Crazy. I can see that wait-and-watch at 2 km or 1 km, but at 500 m? Gilbert looked right, saw Martin coming by, he should have jumped, tried to get on Martin's wheel, get a 200 m lead out then sprint. If someone else jumps and gets on his (Gilbert's) wheel, then they are both being led out by Martin. For Gilbert you can substitute Valverde, etc. The group doesn't start chasing for 200 m or so, until Martin is starting the last turn, by then they knew they were racing for second.
Yes Gilbert should have gone with Martin but he was caught by surprise and hesitated, fatal. Classic move, hang on the back of a medium sized group, wait for the favourites to start marking each other then sprint wide and give it everything.