While gas ovens heat much quicker than electric ovens, they do not do so fast enough to eliminate preheating entirely. If a recipe calls for a pre-heated oven, a gas oven normally reaches the temperature in about half the time it would take an electric stove to heat up.
In many cases, pre-heating is a completely optional step that only wastes energy. Most meats, vegetables and casseroles will come out fine without pre-heating. Breads, pastries, pies, fresh pizza and souffles, on the other hand, require strong heat throughout the cooking process and can come out flat or soggy without a preheat. As a general rule, any recipe involving baking powder or yeast is best placed into a pre-heated oven.