You can do that as follows.
On a computer go to your calendar and click on "manage settings" in the top right hand corner.
Then click on "Availability" and then on the "Edit" button in the "Trip length" section.
You may have to scroll down to see the "Add a requirement for seasons or weekends" link. Click on that.
Then from the "during" drop down box select "Specific dates" and enter the date range for your gap. Then set the minimum to 1.
Select "Save". If you want to set a minium for another date range click on "Add another requirement" and proceed as above.
Select "Save" again when you've set the minumum for all gaps.
You can delete the requirement by clicking on the "X" in the "During x-y guests stay ..." line.
It can also be done on the Android app by going to your calendar, selecting the wheel icon in the top right and then tapping on "calendar settings", "trip lenght" and "Add a seasonal requirement".