If the old testament doesn't tell gentiles about God, but only the Israelites, why do you think you should read it? It would be like reading someone else's mail.
Although all scripture is written for everyone's learning. Not all scripture is written to everyone. Example: Are we required to bring a sacrifice to the Temple?
There were gentile's that left Egypt with the Hebrews. Remember there is NO agreed upon Mosaic covenant at this point.
It's at Sinai & when they ALL agree to do whatever the Lord commands. The Nation of Israel (not a nation while in Egypt) is born. No Mosaic sacrificial system yet. But, everyone that agreed was now Israeli. Every person there becomes, part, of the ONE people/nation God chose to proclaim His name.
After all the laws added in Lev & Deut anyone/person that wanted/chose to serve the God of Israel could become part of His people.
The day ALL agreed to the Lords words, a Nation was born = every person that agreed became 100% Israeli.
Any none Israeli was welcome become a proselyte/convert = no longer a gentile. However they had to:
1) Make a vow to adhere to Mosaic laws.
2) Get circumcised
3) Bring a sin offering/sacrifice to the Temple
4) The final action required: Undergo full/immersion Mikveh: a ritual act of purification, symbolizing a radical change of heart, a total commitment. At this point you're now clean & under Mosaic law. No longer an unclean, bastard, outside of covenant seals.
Gentiles did not observe purity laws (God gave/commanded) So, were considered ritually impure as a matter of course.
Gentiles ate impure foods, came into regular contact with impure substances, and worse, committed idolatry & defiling sexual acts
Find a few verses (there are more) that teach gentiles weren't part of Israel
Matthew 7:6 Jesus said; give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you.
(MY NOTE: Dogs, swine & gentiles all considered unclean under Jewish Law)
John 18:28 Then led they Jesus from Caiaphas unto the (roman/gentile) hall of judgment: and it was early; and they themselves went not into the judgment hall, lest they should be defiled; but that they might eat the passover.
(MY NOTE: Just being in the gentile hall of judgment made a Jew UNCLEAN!)
Acts 10:28 Peter said unto them, Ye know how that it is an """unlawful""" thing for a man that is a Jew to keep company, or come unto one of another nation; but God hath shewed me that I should not call any man common or unclean.
(MY NOTE: Peter tells Cornelius/a gentile. It's UNLAWFULL for a Jew to hang around gentiles)
John 4:9 Then saith the woman of Samaria unto him (Jesus), How is it that thou, being a Jew, askest drink of me, which am a woman of Samaria? for the Jews have no dealings with the Samaritans.
(MY NOTE: Although Samaritans had Jewish roots. They had intermarried (against Mosaic law) & were considered/viewed godless/unclean bastards, living outside of covenant seals same all gentiles)
Eph 2:12 That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world:
(MY NOTE: Gentiles referred to as the Uncircumcision (vs 11) were without God, having no covenant promises & a hopeless future)
13 But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ.
(MY NOTE: But, NOW! = POST death & resurrection/new Melchizedek covenant. Believing gentiles are no longer bastards outside of covenant seals)
Council at Jerusalem
Acts 15:
1 And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved.
(MY NOTE: Jews try to place gentiles under Mosaic law)
5 But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses.
(NOTE: These Christ rejecting Pharisees claimed gentile believers needed to be circumcised & commanded them to keep the law of Moses).
6 And the apostles and elders came together for to consider of this matter.
23 And they wrote letters by them after this manner; The apostles and elders and brethren send greeting unto the brethren which are of the Gentiles in Antioch and Syria and Cilicia.
(MY NOTE: The apostles send letters to all the gentile believing churches in Antioch, Syria & Cilicia.)
24 Forasmuch as we have heard, that certain which went out from us have troubled you with words, subverting your souls, saying, Ye must be circumcised, and keep the law: to whom we gave no such commandment:
(MY NOTE: The letters written & sent by Christ chosen apostles. Telling the gentiles you are not, nor were you ever, required to be circumcised or keep Mosaic law)
Rom 2:14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves
(MY NOTE: WHICH HAVE NOT THE LAW. Gentiles were never under/had Mosaic law)
Acts 22;
17 And it came to pass, that, when I was come again to Jerusalem, even while I (Paul) prayed in the temple, I was in a trance;
18 And saw him (Christ) saying unto me, Make haste, and get thee quickly out of Jerusalem: for they will not receive thy testimony concerning me.
19 Paul said, Lord, they know that I imprisoned and beat in every synagogue them that believed on thee:
20 Paul said, when the blood of thy martyr Stephen was shed, I also was standing by, and consenting unto his death, and kept the raiment of them that slew him.
21 And (Christ) said unto me, Depart: for I will send thee (Paul) far hence unto the Gentiles.
22 And they (this large Jewish audience) gave him audience (they listened to Paul) unto this word, and then lifted up their voices, and said, Away with such a fellow from the earth (A literal riot ensues): for it is not fit that he should live.
(MY NOTE: Paul said (ONE WORD, GENTILE & the Jews wanted to KILL him!)
23 And as they cried out, and cast off their clothes
(NOTE: A custom for men when greatly irritated & their preparing to stone Paul. They threw dust into the air (a sign of contempt see 2 Sam).
Romans soldiers had to come arrest Paul & put him into protective custody)