What is the ruling on selling cooking gas cylinders in a country where the majority of its inhabitants are disbelievers, taking into account that some of the restaurants that will purchase the gas prepare forbidden foods?

It is not forbidden to sell utensils that can be used for cooking unlawful things unless it is known or highly probable that they will be used for that purpose. If the predominant use is unlawful, then selling them is forbidden. This is indicated by the hadith: "Do not sell slave-girls who sing, nor female singers," because since the predominant benefit derived from them was in unlawful acts, their sale was forbidden. Accordingly, it is not forbidden to sell them to individuals and households. As for restaurants, their situation is to be considered based on reality; if the predominant use is known, action should be taken accordingly.