A grill pan, of which there are many brands on the market, is a type of frying pan with raised ridges that is meant to simulate grilling on a rack in the great outdoors. It doesn’t really succeed, but with its ridges, is able to create a pattern on your food as if it had been grilled. The reason it doesn’t succeed is that the food does not come in contact with the flame or even particularly high temperatures, and so does not approach the intensity of flavor we look for in grilled foods.
Most manufacturers recommend you cook on their grill pans at medium heat, to take care of the pan and to minimize smoking and spattering. Burt Wolf, author of The New Cooks' Catalogue: The Definitive Guide to Cooking Equipment (Canada, UK), says he has found that grill pans work well for firm vegetables, shrimp, pork chops, hamburgers, hot dogs, oily fish and thin steaks. With thick steaks, the outside is too often overdone before the center is cooked.
The grill pan does not care what your source of heat is - either gas or electric will work equally well.