https://www.uscis.gov/sites/default/files/USCIS/Resources/B5en.pdf
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What will happen if I do not apply for a reentry permit
before I travel outside of the United States?
If you are an LPR planning to travel outside of the United States for
1 year or more, it is important that you apply for a reentry permit
before you depart the United States. If you stay outside of the
United States for 1 year or more and did not apply for a reentry
permit before you left, you may be considered to have abandoned
your permanent resident status. If this happens, you may be
referred to appear before an immigration judge to decide whether or
not you have abandoned your status.
If you are in this situation, contact the U.S. consulate about
a returning resident visa.