The gap occurs when the opening price is higher or lower than the previous closing price.
For example, if the previous closing price was 100 riyals:
Ascending gap: the stock opens at a price of 101, for instance.
Descending gap: the stock opens at a price of 99, for instance. 
Explanation of types of gaps in technical analysis:
Closed ascending gap: the stock opened at a price higher than 100, then the price dropped to the same or lower than the previous closing price, meaning a minimum price that is less than or equal to 100.

Closed descending gap: the stock opened at a price lower than 100, then the price rose to be higher than or equal to the previous closing price of 100.

Ascending list: the minimum price is still higher than the previous closing price of 100, and the gap remains.
Descending list: the maximum price is still lower than the previous closing price of 100, and the gap remains.