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.

In some ways, what you're asking is the inverse of What can I use for a Crème brûlée if I don't have a blow torch? . Of course, not everyone has a blow torch for this sort of thing.

For lasagne, if you normally do the 'cook covered 'til heated, then uncover & broil' ... do the opposite. Cook it uncovered 'til the top is brown. If it's not hot enough in the middle, cover with aluminum foil and return to the oven 'til it's done.

You can simply cook the food a bit longer to get that crisp crust on top - it is possible that the dish will be a touch drier for being baked in regular oven heat until it browns, instead of just the top element heating up in a broil (while the rest of the oven can cool a bit) - but I don't think the difference will be huge, just perhaps add a bit more moisture and be aware that all the crusts will be a touch thicker.

You can, as Joe mentioned, cook lasagne style - with foil over the dish for part of the time for slower cooking, and uncovered part of the time to encourage that crust to form. Just like with baking bread, a crust will form because the outer layer is exposed to radiant heat, and it dries a bit more, and you get browning from Mailard reactions and Caramelization.

Again, the difference between it cooking that much in the mostly indirect heat of the oven vs intense directed heat on the broil means it might take a little longer, and the crust might be a bit thicker (as the dish cooks a bit extra from the outside in), but it is still quite doable. It doesn't really matter much if the dish is browned first or last - though it might be a bit moister at the surface if it's browned first (the dish's steam trapped under foil), and the possibility of overcooking it and perhaps a bit drier if crisped at the end, and a bit more control over how brown it gets - but those are.

In general, you can get a crisp crust and browning even without a broil function - I have even managed it with the oven off (and therefore only residual, virtually no radiant heat), when I'm making tortilla pizzas. Usually, the oven is preheated, pizzas are quickly assembled just before getting tossed in to cook and pulled when done, and the oven gets turned off halfway through the second (when the cheese just starts melting) with the crisping and browning of the second, and any subsequent pizzas (sometimes two or three more) all happening with the oven off - and they can still get very crisp and brown, it just takes a bit longer.

The point of that anecdote is, you can still get a nice crisp crust on top. It takes a bit more time, since the dish has to cook through instead of only getting the top heated. It may let the food cook a little bit more around the edges - which I usually find controllable by making a bit wetter, or dripping water to cool and slow any overcooked bits. But it is usually fine, not a huge deal if a broiling setting isn't used or isn't available.

If your gas oven has heating elements just on the bottom of the oven, maybe placing the whole dish on a baking sheet with a bit of water poured on it will cool the bottom and slow its cooking enough to let the top brown before the bottom overcooks. If your oven has heating elements from the top instead or as well, or around the edges, the oven should cook more evenly - and the top will have its opportunity to brown as the dish cooks from the outside in (as ovens do).