Whoever is steadfast in righteousness will live, but whoever pursues evil will die.
Life and death, righteousness and evil. These have been with us throughout the proverbs.
However, this does give an answer to the question of verse 18. What is the gain and reward. The gain and reward is life. The righteous will be rewarded with life, but the evil will gain nothing. The absolute of nothing is death.
Still, the choice of life and death always takes me back to Deuteronomy 30.
11 Surely, this commandment that I am commanding you today is not too hard for you, nor is it too far away. 12It is not in heaven, that you should say, ‘Who will go up to heaven for us, and get it for us so that we may hear it and observe it?’ 13Neither is it beyond the sea, that you should say, ‘Who will cross to the other side of the sea for us, and get it for us so that we may hear it and observe it?’ 14No, the word is very near to you; it is in your mouth and in your heart for you to observe.
15 See, I have set before you today life and prosperity, death and adversity. 16If you obey the commandments of the Lord your God that I am commanding you today, by loving the Lord your God, walking in his ways, and observing his commandments, decrees, and ordinances, then you shall live and become numerous, and the Lord your God will bless you in the land that you are entering to possess. 17But if your heart turns away and you do not hear, but are led astray to bow down to other gods and serve them, 18I declare to you today that you shall perish; you shall not live long in the land that you are crossing the Jordan to enter and possess. 19I call heaven and earth to witness against you today that I have set before you life and death, blessings and curses. Choose life so that you and your descendants may live, 20loving the Lord your God, obeying him, and holding fast to him; for that means life to you and length of days, so that you may live in the land that the Lord swore to give to your ancestors, to Abraham, to Isaac, and to Jacob.