Dead, it would appear.
I love a good back story – it gives a show perspective. If you’re looking for some history on your favorite characters, check out this link to the “official” Gossip Girl wedding announcement.
Take special note that:
The bride’s previous three marriages ended in divorce. The bridegroom is a widower.
So let’s speculate (what fun!): What do you think happened to Chuck’s mom? The wilder, the better.

she probly overdosed.
haha.
OOOOOH, HOW JUICY!
CAN’T WAIT ‘TILL I FIND OUT. HMMM..
I BET YOU ITS SOMETHING
TRAGIC LIKE SHE DIED WHEN
CHUCK WAS YOUNGER
DURING LIKE CHILDBIRTH
OR A CAR ACCIDENT.
I Don’t know…hmmm. Overdosing is a possibility, as well as a car accident, or maybe it was something long and tramatic like cancer? Or maybe it was…suicide.