"Were strictly regulated". You are giving the time of the action: "a few years ago". Therefore the simple past is required.
However, this sentence strikes me as a bit odd. (I don't know where it comes from.) It suggests that although a few years ago gas emissions were strictly regulated, they are not regulated now. Are they still regulated? If so, a better way to put the sentence would be, "A few years ago strict regulations on gas emissions were put in place."
