If your last trip outside the US was under a year, but you get in trouble at the POE because of the accumulation of absences, if they take away the card they would let you in but tell you to see an immigration judge to plead your case and try to get it back. If you refuse to see the judge, or if the judge deems your permanent residence to be abandoned, you'll be deported or told to leave by a given date.
But if your last trip outside the US was a year or more and you don't have a Reentry Permit or Returning Resident visa or military paperwork, they'll probably refuse entry because the green card is not a valid entry document by itself if you have been gone for a year or more.
EB3 ROW I-485 Approved: July 2007
USC: July 2013
I am a layman, not a lawyer. What I write here is not official or professional legal advice. In addition, my answers on this forum are specific to the scenarios discussed in each thread and should not be generalized to other situations.