If this 9 months is your only long absence and there are no aggravating factors (e.g. you were working abroad for a non-US company, or you accepted permanent resident status in another country), you'll almost surely keep your green card. But be prepared to be harassed at the port of entry, and they might give you a warning (and record the warning in the system, so you could lose the GC if you take another long trip in the near future).
