There you will find descriptions of the Bottles with their tall and narrow neck. You will also find the story that there was no other cap but a plug that was built by the yeast all by its own.
This will make you probably think of a pellicle as it is formed by oxidative yeasts and bacteria. But they tell us it was highly carbonated this way, and there is no reason to think it was held very cold all the time. Maybe it was only a weak pellicle unable to hold any pressure but I am pretty sure that this plug was there in every bottle.
You may also notice that the neck of the bottle is well suited for minimizing oxygen uptake as the surface of the beer is minimal.
Wilhelm Henneberg (1871-1936) was a german bacteriologist and studied in Halle (close to Leipzig in the Gose-region) and was an expert in yeast, lactic bacteria and acetic bacteria. In 1897 he discovered an acetic bacteria in Döllnitzer Gose that was named by himself "Bakterium acetosum". That bacterium produced acetic acid in a quality that made it suitable for vinegar production.
Might this species be able to produce a polymer that is durable enough to build a plug in a bottleneck which can hold enough pressure for carbonization?
Maybe it was just a normal acetic bacteria as they were found in these days nearly in every bottle of beer. But then what build the pellicle?
Is it possible that an acetic acid bacteria builds a strong pellicle that limits its own oxygen uptake in order to stop the conversion to vinegar?
One would have to try and get more information on the bacterium.