You must renounce your US citizenship in a US Consuate outside US. Thus, you need to get a visa to enter US. That means you cannot return to a green card holder automatically unless you apply for it again. Read the following at:
Why Some Americans Renounce U.S. Citizenship | CitizenPath
Determine Your New Home
You must renounce U.S. citizenship at a consular office outside the United States. Although you don’t have to select a consular office in your new home country, most people choose to do so out of convenience. It is also highly recommended that you obtain legal status and a passport in your new home country.
Read the following at: Renounce or Lose Your U.S. Citizenship | USAGov
Giving up your U.S. citizenship has consequences. You should never make this decision lightly, as it can only be undone under very limited circumstances. Renouncing your U.S. citizenship means that you:
- Give up your rights and responsibilities as a U.S. citizen.
- Must become a citizen of another nation, or risk becoming "stateless."
- May need a visa to visit the United States.
Watch the following video for how to reenter US after renouncing US citizenship:
3 ways to visit the US after renouncing Citizenship - YouTube
