Gratin without a broiler
Okay, as mentioned in the comments, the easiest way is to use a broiler, but if your gas oven doesn't have one.....

When following the instructions for thin crust pizza from Cooks Illustrated (on a pizza stone in my oven), one thing they suggested to help the cheese melt and crisp a bit was to NOT put the pizza on the lower rack.

They figured the oven temperature was going to be high enough to cook the food, so no need to put it closer to the heat source that was below.  However, by putting it on the closest rack to the top that would still fit the food, there was some additional heat radiated back to the top of the pizza that was absorbed by the metal roof of the oven.

Whether that works well enough for your purposes or not, I don't know, but it seems to be a similar concept.