As far as I know, Gibson never actually made resonator guitars until they bought the rights to the "Dobro" name in 1993. It's quite possible some resos were made for Gibson by National, Dobro, Regal or whoever. That sort of job-work was pretty common in the first half of the 20th century and relations between instrument companies were a bit incestuous. If Gibson only had a limited demand for resos, it would have been cheaper and easier to order a few unlabeled instruments from a competitor and stick a Gibson decal on them than to tool up to make them.