You can't "Force" it, but you can slowly lower the temperature in their cages down to about 70 degrees over about a 2 week period. Then slowly raise the temps back up after a few weeks. (Not sure what would be the exact temps and timeline, since I don't do this).
This will mimic their Winter schedule in the wild, and usually after the winter schedule they will want to breed.
I am sure someone else will have more info about the exact Temps, etc.
A Leopard geckos natural time to breed is January to june/july. Most of the country gets enough of a temp drop in the winter to replicate their natural cycle, so most people don't do anything special for their geckos to breed. But as mentioned above you can do this your self. first make sure your gecko is nice and fat. Then stop feeding him for about a week before you start dropping the temps. This way there should not be any food left in the stomach to rot. Slowly drop the temps to about 70-75 degrees over a period of a couple weeks. Leave it at these temps for about 10-14 days. Then slowly bring the temps back up to 85-90. You can start feeding again at 85 degrees. Make sure that fresh water is always available. This should take about a total of 1 1/2 to 2 months.
Well a natural process would be about 3 months cooling and some breeders do that at the end of the season after they have fattened back up to simulate natural process others don't.
Stimulating breeding and simulating the natural process are 2 different things.
I did it with the males and the females in groups, they were all adults virgins so I cooled them to stimulate breeding not to simulate the natural cooling in the wild.