In this case, I think the best option is "cocina a gas" to avoid the confusion with "de" gas (meaning made of).
I feel that the Gallicisms show more in expressions like: "a" hacer (preposition "a" + infinitive verb), like in French: "à faire".
Even if "a gas" should be avoided as Gallicism, it's the best option for the reader in order to avoid confusion.
So our customers will prefer to avoid confusion, I think.
