As a permanent resident of the United States you may travel abroad for periods of less than one year without additional documentation. You will need valid proof of permanent residence (unexpired green card) to reenter the U.S.
If you plan to travel for a period of more than one year, you must obtain a Re Entry Permit. Although you can have the Re Entry Permit mailed to an alternate international address or U.S. consulate office, it is recommended that you do not travel until you have received it.
When you file your Form I-131 to obtain a Re Entry Permit, USCIS will notify you when to appear for your biometrics appointment. The biometrics appointment must be conducted in the United States. If you leave the United States before you provide your biometrics, USCIS may deny your application.