I don't think liquid smoke is super hard to find....I lived in a tiny town last summer and the Fareway had it, I think it's normally by the barbecue sauce. I use Wright's brand, which doesn't specifically say gluten free on the label, but the only ingredients are water and natural hickory smoke concentrate, and I haven't had any issues with it. Also, according to their website, Colgin brand natural liquid smoke is gluten free (I just haven't had any luck finding it).
I can't think at what point in the manufacturing process gluten would come into play; I remember looking it up to see whether liquid smoke or smoke-smoke engendered higher levels of acrylamide after I heard something on the radio about smoked food... IIRC, liquid smoke is produced by bubbling smoke vapour through water. Unless they're making burnt offerings of loaves and young calves, I think we're good.
Unless they've started making smokey soy sauce or something while I wasn't looking.