Gas mains are pipes which supply gas to buildings for heating and other purposes. Therefore "main supply gases" are gases which are sent through those pipes, or mains as they are known.
The sentence is therefore talking about controlling and maintaining procedures which ensure the safety of gases used for heating, welding/burning, whether they come through a pipe or whether they are stored in a cylinder.
The original sentence is fine. "Is to" is being used in an emphatic way to mean "must".
"The company must control and maintain procedures to assure the safety of main supply or bottled gases used for heating and gas welding / burning."
Does that help?
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