First of all, what is your idea of observing the Sabbath. God said to keep it separate, and this I know I do.
Secondly, why do you refer to a "gentile christian?" Do you not know? Are you not aware that we are fellow heirs now
with Israel? Our Kingdom to come has a capital called New Jerusalem, did you not know this?
If you believe you are not of the heirs with all Hebrews of Israel, then you should study the Word much more.. It is all to easy to p pick and choose a loose line here and another there to cut and paste to support an argument of the mind, but to be accurate, truly, we learn from thh Holy Spirit.
Now as for how your type of believer should or should not observe the Sabbath, that is between the believer and His Father. I do not condemn anyone for observing a Sabbath onthe first day, but I do glorify my Father by observing it on the day He sleected and designated.
It is never a sin to teach obeience of our Father, and all who know Jesus Chrsit are perfectly capable by the HOly Spirit to discern which laws are to be obeyed. Do not act as though this is not tru for it is our faith that establishes the law, that is according to Paul in Romans.
God bless you and do obey theFather always. I cannot advise any to disobey Him...that would be Satan talking.
Let me ask you this question then:
Is a Gentile Christian who does not observe the Sabbath from Friday night until Saturday as outlined in the Law of Moses - are they sinning and dis-obeying God?