Shalom.
You can understand the gospel by reading only the new testament. I do recommend that a person reads the Torah or the TaNaK as a foundation first. However, this may be in study as the gospel is of first priority. So long as a person can sufficiently understand it. However, the new covenant for the house of Israel and the house of Judah, God's law written on minds and hearts, was first spoken of in Jeremiah (Chapter 31). As for if Gentiles can be included I recommend that you read 1 Corinthians 11:25. If 1 Corinthians was written to Corinthian believers, Jews or Gentiles.
Acts 15:29 NASB - that you abstain from things sacrificed to idols and from blood and from things strangled and from fornication; if you keep yourselves free from such things, you will do well. Farewell."
Romans 8:21 NASB - that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God.
2 Corinthians 7:1 NASB - Therefore, having these promises, beloved, let us cleanse ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of God.
Shalom.
Jacob