A cast iron grill pan differs from a regular frying pan in that it has raised, parallel ridges covering the cooking surface. The ridges mimic a metal grill, both by allowing fats to drip away from the food and by creating grill lines on the food's surface. If a recipe calls for a grill pan and you don't have one, there are several other things you can use as a grill pan alternative.
Frying Pan or Griddle
The only difference between a grill pan or stove top grill and a frying pan or griddle are the ridges. While some foods, like vegetables, will cook differently in a flat pan, a regular frying pan can be used instead of a grill pan for things like sausages, burgers and chicken. At the end of the cooking time, transfer the food to a plate lined with paper towels, a step that's not necessary with the grill pan, to absorb the excess grease.
Electric Countertop Grill
An electric countertop grill works in the same way as a grill pan. Instead of using a stove top burner to heat the ridged surface, the surface is self-heating. Most electric grills use a timer that cuts off the heat when the time is up to avoid overcooking food. Electric grills also commonly close, so their two surfaces heat the food simultaneously. Adjust the time to account for the top surface, or follow the recipe using the bottom surface only of the open grill.
Broiler Substitutes for Grill Pans
The broiler in your oven is also an alternative to a grill pan for foods that will saute in a frying pan, such as vegetables. It's also a substitute for cooking grilled chicken, steak, salmon and kabobs that will give it a charred surface -- basically, it's like grilling from the top instead of the bottom. When substituting the broiler for a grill pan in a recipe, do not rely on the recipe's cooking times. Check the food frequently as it cooks, and turn it as you would with a grill, so it cooks evenly.
Outdoor Grill Alternative
While a grill pan is really a substitute for an outdoor grill, an outdoor charcoal or gas grill can also be used when a recipe calls for a grill pan. The cooking temperature of an outdoor grill may vary, so adjust the cooking times. Don't cover the grill when following a grill pan recipe. Instead, place the food on the grill over direct heat and turn it periodically.