Gas props for topper doors. Weather you have a fiberglass topper or aluminum topper Monroe Truck & Auto Accessories has these instock. There are many different kinds of gas props. We highly recommend that you bring your fiberglass topper in to us so we can make sure we get the right prop for you. Most range in size and we measure center to center holes on the gas prop. The poundage or pounds for the prop strengh will also vary. Most are measured in pounds but a lot of the gas prop manufactures come from overseas. Stabilus gas props, Suspa gas props are made in michigan, and other brands of gas props.