Any trip over 6 months affects your timeline for naturalization. Every time you spend over 6 months overseas in one trip, you're breaking continuous residency (with some exceptions).
This means, you start counting 3 or 5 years for naturalizing not from the time you became a resident, but the date you came back to the US from 6+ months trip.
There's nothing particular about 9 months, but if you stay overseas for over 12 months, you risk losing LPR status. Upon entry to the US, it's possible the officer would offer you to give up GC voluntarily by signing I-407 OR refer you to immigration court.