Regard him as a gentile and tax collector

"Regard him as a gentile and a tax collector." --Jesus (Matthew 18.17)

What did Jesus mean by this concerning a brother who sins against another and can not be "won back?”

I believe the consequence here is the same as the one Apostle Paul states in 2 places in the New Testament concerning a sinning brother, but the audiences are a little different and the situation is different, but the meaning of the consequence is the same but implied in Jesus' words while Paul's are straight forward.

Jesus says,

"If your brother sins against you, go and confront him while the two of you are alone.

If he listens to you,

you have won back your brother.

But if he doesn't listen, take one or two others with you so that ‘every word may be confirmed by the testimony of two or three witnesses.' If, however, he ignores them, tell it to the congregation. If he also ignores the congregation,

 regard him as a gentile and a tax collector."

Matt 18.15-17 ISV

Consider that Jesus is speaking to his disciples who are Palestinian Jews.

He instructs on how to deal with a brother--a fellow believer among them--if they sin against them.  The offended party is to privately confront the brother.  If they admit to the deed, they are won back.  Fellowship is restored between them.  But if not, then the offended party takes another believer with him to confront the other.  If that fails, then the congregation (assembly of believers) gets involved.  If that fails, then the offender is to be regarded or "like" (NKJ) "a gentile and tax collector."

Why treat them like a gentile (non-Jew) and tax collector?

 What is the implication of this consequence?

The Jews--especially Palestinian Jews-- did not keep company with Gentiles, and they didn't keep company with tax collectors.  What's wrong with either?  Jesus didn't say regard them as unsaved people or non-believers.  We know in the Gospel accounts that there were both Gentles and Tax collectors who were believers in Christ--even the Matthew who wrote this Gospel was a tax collector.  Jesus was just appealing to the common practice among Palestinian Jews of not keeping company  with these people.

"They [tax collectors] were reviled by the Jews of Jesus' day because of their perceived greed and collaboration with the Roman occupiers." Wikipedia

I believe Apostle Paul says the same thing.  He writes to not keep company with a brother who is committing certain sins.  It isn't to consider them unsaved but don't keep company with them.

"But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person."

1 Corinthians 5.11

Paul also says,

“But we command you, brethren, in the name of our Lord Jesus Christ, that you withdraw from every brother who walks disorderly and not according to the tradition which he received from us.  For you yourselves know how you ought to follow us, for we were not disorderly among you…”

 "And if anyone does not obey our word in this epistle, note that person and do not keep company with him, that he may be ashamed. Yet do not count him as an enemy, but admonish him as a brother."

2 Thessalonians 3.6-7, 14-15

Is the brother who does not admit to an offence to the church worse than the brother who is sexually immoral or walks disorderly that he is to be considered unsaved?

Peters words to Cornelius would also support the idea that to treat one as a gentile is to not keep company with them (the consequence Paul gives concerning the disorderly brother):  “You know how unlawful it is for a Jewish man to keep company with or go to one of another nation.” Acts 10.28

So I would maintain that Jesus was saying the same thing as Paul.  To a Jewish audience, especially the Palestinian Jew, the gentile and tax collector were those one did not keep company with.  And this is what Paul said to do with a brother who was in a sin.

How is a brother to be regarded/ like a gentile and tax collector? It is by not keeping company with him.  

But the aim in this is that they might be ashamed, admit to their wrong, and be "won back" ( restored to fellowship).