What happens when you believe and stand on the Gospel?

Believing and standing on the Gospel saves you. It keeps you from being blown over by every wind of doctrine and false teacher. Standing on this Gospel means you need not worry about eternity, because it is your salvation, saving you from eternal doom.

Les explains that the Gospel is the one he preached, which the Corinthians received and wherein they stand. When you believe it, you are not blown over with every little wind of doctrine and every false teacher that comes along. You stand on this Gospel, and you won't have to worry about eternity. By it you are saved. Les emphasizes that 'saved' is a Scriptural word, not a denominational one, and he uses the analogy of a bankrupt man rescued by a rich uncle's inheritance to illustrate salvation—being saved from something. We are saved from an eternal doom. This Gospel is our salvation. He warns that if you've kept in memory what he preached, lest you believe in vain, know what you believe.

1 Corinthians 15:1

Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;

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Is salvation by the Gospel conditional, and what happens if you do not understand what you believe?

Yes—salvation is by the Gospel, and it is conditional: you must understand and know what you believe, or you have believed in vain. The Gospel is that Christ died for our sins, was buried, and rose again the third day according to the Scriptures.