If we have to gain the inheritance by following those laws, then it no longer comes to us because of the promise. However, God freely gave the inheritance to Abraham through a promise.
Certainly, there is a curse on all who rely on their own efforts because Scripture says, “Whoever doesn’t obey everything that is written in Moses’ Teachings is cursed.”
The promises were spoken to Abraham and to his descendant. Scripture doesn’t say, “descendants,” referring to many, but “your descendant,” referring to one. That descendant is Christ.