Gather Wait sets the units to 'wait' until the group has finished the previous command.

As an example:

1. Move to location A
2. Gather Wait (wait until all units have gathered at location A)
3. (Shift) Move to location B

So all units that arrive early at location A, will wait/pause there, until every unit (that got the Gather Wait command) has arrived.

Only then, all units will automatically continue with the next queued command (for example Move to location B).