Hmm. After having just finished the author's more recent manga, Shimeji Simulation, I have to say I think I agree with Mast_Chacha's interpretation of the ending. In Shimeji Simulation, we find out that the world is a virtual simulation stored in a supercomputer floating in space that holds the souls of all humanity after a plague and then a great war. Additionally, in the manga we see Chito and Yuuri in the world. While this could just be a throw-in for fans of Girls' Last Tour, the stories of both of the worlds seem to line up well. The plague and then great war happened, throwing humanity into a death spiral which they escaped by virtualizing their souls and heading off into space, which is why in GLT we don't see any corpses around the world. This would also be supported by the fact that in the GLT chapter with the rocket we see that the third rocket launched by humanity is still out there in space, somewhere far off. Then, at the end of GLT as Chito and Yuuri finally reach the highest point in the city, they come across the mysterious black stone that's mentioned, shown to contain machine code, but never elaborated upon. I would say that's most likely a transmitter, considering it's the only structure so high up. Chito and Yuuri, after waking up from their sleep, decide to investigate what the thing is (as we can see by the fact that in the final frame of the Volume 6 extra chapter there's a good deal of the face of the structure that has been wiped clean of snow), activate it, and have their souls virtualized and sent to the spacecraft, which is why they show up in Shimeji Simulation. I could very well be wrong about this and just hoping that the two girls survive, but at the same time it seems plausible to me that this could be the intended chronology.