If you are a green card holder and will be outside the US for less than a year, you will generally not need Form I-131. However, if you will be traveling for about one year, you may want to get Form I-131. If you will be traveling for longer than a year, you will need Form I-131 to re-enter the US. It is important to file Form I-131 before leaving the country, so that if you expect your trip may take longer than anticipated, apply for Form I-131 just in case.
There are other situations in which you may need Form I-131 as well. For example, if you are a permanent resident of the US but do not have a passport from your home country you will need to file Form I-131 before leaving the US. As well, if you are an asylum seeker or refugee you may also need Form I-131 if you plan to leave the US.