Retention gap or guard column is a length of fused silica capillary with no chromatographic phase covering its internal surface. When working with samples containing non-volatiles or slightly volatile compounds in split injection mode, the guard column can trap these compounds so that they don't contaminate the analytical column.

When working in splitless mode, the retention gap allows for the injection solvent and analytes to condense, as the temperature program starts the injection solvent will vaporize and leave the column, while the analytes focus at the head of the analytical column and then progress down the column. So physically they are the same, but are given different names depending upon their use.