Salvation is always by grace thru faith at all times I agree, but you show your faith in God by following what he commanded you to do, and that differs depending on the time period in question.
There has never been two gospels, but one and the same that was preached to Abraham was also he message of rhw Messiah from the beginning and the very same one through which we are saved: all who put their trust in Him, they will not be ashamed.