The flex gas line diameter shall be 3/8″ and the length shall not less then 6 feet. The 6 ft. length is so the dryer can be slid forward to clean and work behind it without having to remove the gas line flex.
The gas supply pipe extending out of the wall should be 1/2″. This means you’ll need a 1/2 by 3/8 shut-off valve for that pipe. The shut-off’s female end attaches to the pipe while the flex line attaches to the male end of the shut-off valve.
This diagram from the People’s Republik of Vermont Extension Service might help, too.
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