There is no law that "a green card holder can not stay outside of USA more than 6 months". There is a regulation in 8 CFR 211.1 on what documents are acceptable for a returning permanent resident. For the purposes of this, a green card is only an acceptable document for re-entry after an absence of less than 1 year.
There are some other acceptable documents; for example, a Re-entry Permit is valid for re-entry during the validity period of the permit, which are usually issued for 2 years. But Re-entry Permits can only be applied for while you are in the US, so if you didn't get one, it will not help you now; and if you did get one, it is probably expired because you were away so long.
Another option is to apply for an SB-1 returning resident visa at a US consulate abroad. This is an immigrant visa that would allow you to return to the US. However, US consulates only issue SB-1 visas if you can show that you could not return any earlier due to circumstances beyond your control. This is a very high bar to meet.
Or, if your green card is unexpired, you could simply fly with your green card to the US and try to seek entry. Although you technically don't have any acceptable documents for re-entry, since you have been outside the US for more than 1 year, and don't have a Re-entry Permit (or had a Re-entry Permit that expired), and don't have an SB-1 visa, the immigration officers at entry have the power to waive your failure to present an acceptable document for re-entry and let you in directly (potentially with a warning), if they determine that you have not abandoned residence. Many people report that they have success getting in, especially if it's the first time they've stayed out for more than 1 year, and it's not too much more than 1 year, and especially recently with the COVID-19 situation, but your absence does seem a little bit long, so I don't know what the chances of success are for you.
If the immigration officer doesn't decide to let you in directly, they might try to have you voluntarily sign form I-407 to voluntarily relinquish your green card. They might pressure you by saying that if you don't sign, they will deport you and ban you (which is half true but not as scary as it sounds; see later). I suggest that whatever you do, you refuse to sign the form (it is voluntary) and refuse to voluntarily depart. The immigration officer does not have the power to revoke your permanent residency; only an immigration judge in immigration court can do that. So what the immigration officer would do if you refuse to voluntarily relinquish your green card is give you a Notice to Appear for removal proceedings in immigration court at a later date. During this time, you will temporarily be in the US, and you can find an immigration lawyer or represent yourself to the immigration judge, to argue why you didn't abandon residence (think of the removal proceedings as a kind of "appeal" of the immigration officer's decision).
Abandonment of residence is based on many factors (see a summary of case law here), and not on the length of absence alone. I don't know how likely you are to succeed, given that your absence is a bit long. If the immigration judge agrees with you, the immigration judge can waive your failure to have an acceptable document for re-entry and let you in. If the immigration judge rules against you, you will be ordered removed. You can appeal to the Board of Immigration Appeals if you wish. Removal will trigger a 5-year INA 212(a)(9)(A)(i) ban, but you can try to request Voluntary Departure from the immigration judge, and if it is granted, and you depart according to its terms, then you do not have this ban.
I believe that the chances of success with the immigration officer and immigration judge are probably higher than the chances of getting an SB-1 visa at a US consulate. You have multiple levels of appeals and can have a lawyer represent you if you get to removal proceedings, whereas you have no recourse for a visa denial at a US consulate.