Advisor getting married, should grad students chip in for gift?
I laughed here; can't imagine my supervisor getting married.  It is just nice to see the supervisor involving in life..  

For the question, Why not? It is a big day for your supervisor. Just make sure the gift come from group of students not only one student. If you missed the first day after his return (assuming he's going on vacation for marriage), then no need for a gift. Either ways, a verbal congratulations is a must here :)