Permanent residents of the United States may travel abroad after they get their Green Cards and they may use their Green Cards to travel abroad for a certain period of time. However, a Green Card holder is likely to lose his Green Card, if he remains abroad for a long time and if he does not wish to remain in America permanently. Many permanent residents have a notion that visiting the United States once in a year is sufficient to retain their lawful status in the country, which is not true. You may travel abroad for less than a year with your valid Green Card but if you have plans to remain in a foreign country for around two years, you must obtain a re-entry permit and you need apply for this permit while in the country.
To obtain this travel document, you need to file Form I-131, Application for a Travel Document and you should appear for biometrics. You will be granted a re-entry permit if you are eligible and this permit is good for 2 years and you must get back to the United States ahead of the expiration date of this permit. This permit is similar to a visa and you need to provide this permit at the port of entry along with your Green Card. However, you will be permitted to get into the country only after immigration inspection and after the US immigration officers make sure that you do not have plans to relinquish your lawful status.
You may not get into trouble if you remain abroad for less than 180 days and your Green Card will help you to get into the United States. But if you remain abroad for more than 180 days, you need to prove that you do not have an intention to live permanently in a foreign country, to get into the United States. You might lose your lawful status, if you remain in a foreign country for more than a year. However, you may not get into trouble if you have strong ties in America and if you own property in America. Immigration officers will consider such factors and grant you entry into the country and in this case, you will have to obtain a returning resident visa to get into the country.
You may apply for and obtain a returning resident visa from an overseas US consulate or embassy, if you had to remain in a foreign country due to an unexpected illness or if you had to remain abroad with a sick family member. You will be permitted to get into the United States with this returning resident visa if you had remained abroad for more than a year without a re-entry permit, due to an unforeseeable circumstance. Nevertheless, you may lose your Green Card, if you are ineligible for a returning resident visa.