Rom 2:14-16 NKJV - (14) for when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, (15) who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves [their] thoughts accusing or else excusing [them]) (16) in the day when God will judge the secrets of men by Jesus Christ, according to my gospel.
Their conscience will still tell them they are doing wrong, and won't be at peace. Their understanding can acknowledge they are doing wrong, or can justify itself as "a necessary evil" or can simply be deceived thinking that they are doing is actually good, or at least not evil. However, they are still guilty, because of their conscience, and their conscience will bear witness against them in the day of judgment, even if they didn't know the law of Moses.
Because of the corruption in the heart of man, namely their understanding and will, God always gave commandments and laws to those that drew near to Him, even before the Law of Moses. The Word of God brings clarity to the understanding that has been darkened because of Sin.
Enoch walked with God:
Gen 5:24
Noah walked with God:
Gen 6:9
Abraham was called to repent and walk with God:
Gen 17:1
Now, all those passages were written by the same author: Moses. What did "walk" mean for him? Let's look at the places where the lifestyle of those that walked with God is explained beside worshiping and praying:
Gen 26:5 NKJV - (5) "because Abraham
obeyed My voice and kept My charge, My commandments, My statutes, and My laws."
Well that verse implies that Abraham received commandments from God
beside the call to leave his people, even though, the Bible does not tell you exactly what were those.
I'm telling that it is totally reasonable to think that all people that sought God before Moses, soon found out God wanted them to obey commandments, laws, regulations, and statues.
Psa 119:105 NKJV - (105) NUN. Your word [is] a lamp to my feet And a light to my path.
Psa 119:130 NKJV - (130) The entrance of Your words gives light; It gives understanding to the simple.
Psa 19:8 NKJV - (8) The statutes of the LORD [are] right, rejoicing the heart; The commandment of the LORD [is] pure, enlightening the eyes;
Adam had one commandment, and the rest was just part of his nature of not sinning. We all need detailed commandments because of our understanding's tendency to get corrupted. We got knowledge of good and evil, and a tendency to choose evil. That's why you need a lot more than just "love your neighbor as yourself" because quickly you will deceive yourself of what really means to love your neighbor as yourself.
Jer 17:9 NKJV - (9) "The heart [is] deceitful above all [things], And desperately wicked; Who can know it?