The gulf is a great chasm or fixed interval separating two places. In the relevant biblical passage it represents an impassable separation after death.
In Jesus’ account of the rich man and Lazarus, a great gulf separates the righteous from the wicked. The image is ethical: the difference lies in their characters, and the separation cannot be crossed merely by a change of condition.
The image resembles rabbinical descriptions of separation between compartments of Hades, but the passage gives the gulf a figurative and ethical significance.