PRS repetition time gap is the slot offset between two consecutive repetition indices of all the PRS resources in a PRS resource set.
In simple words, it is the time difference of a PRS resource and the repetition instance of the same PRS resource.
Consider a PRS resource set has only one PRS resource and that PRS resource has a repetition factor of 2. Assuming PRS resource set period is 8 slots and then PRS resource starts at an offset of 2 slots (slot #2). As the PRS resource repetition is 2, there will be two instances of PRS resource in the PRS resource set period. The first instance is at slot #2, since it is at an offset of 2 slots. Now, we need to figure out at which slot in the 8 slot period, is the repetition of first instance should be. This is known with the help of PRS repetition factor time gap.
If the PRS repetition factor time gap for the above example is 4, then the repetition of first instance is at a gap of (4-1) slots with respect to the first instance. Implies gap of 3 slots from slot #2, which is slot #6. See the below illustration with code comments.
The illustration above indicates only the PRS presence in the slot. But, the actual transmission of PRS in a slot also depends on the muting configuration.
Hope this helps.
Edit: Changed the illustration to look better in the answer.