Girl's Last Tour is a difficult story to fully describe, as the value of it as a story is almost totally dependent on the reader.
Plot is present in the story, but in a different way than most. There is the overarching story of the two girls trying to reach the point that their father told them about, but it is definitely a story where the journey means more than the destination. We spend all of our time exploring the world alongside the two main characters, watching them go through day by day trying to find food to eat, discovering new things and meeting the occasional person or robot to talk to. A lot of the story is absent of any constructive dialogue. Just images of where in the world the girls find themselves in. It leaves room for immersion and for the reader to fill in the gaps.
The characters that appear usually represent different themes and ways of life, with dedication to what you love, or purpose, being a major repeating factor. In a sense, you could say that that is exactly what this story is about- finding your purpose and holding onto it for dear life. The map maker was willing to die for his creations, and the same goes with the airplane maker. The robot that loses its purpose begs for death and an end to the meaningless life it is now living. For the main characters themselves, their purpose is to just live with one another and to stay together. This is so clearly seen in the last scenes of the manga, which I won't spoil, but just know it adds a very poignant underscore to the theme.
The art is super funky in this, so if classic manga drawing is your style, you might not enjoy this, but it is always interesting to see something different.
Overall, what you get out of Girl's Last Tour is dependent on what you put in into thinking about it. For me personally, I didn't actually enjoy the story all that much, and I was left at the end thinking, "Well what was the point of that?" To some it may seem like a story trying to be 'fake deep', while others may pick up many, very real lessons. To me, I clearly see the theme of finding and holding on to what you love, but others may see it as more of a story about the importance of being content, a warning on the environmental state of the world, or about nothing at all, only two girls living life. Either way, it's a fast read that you could go wtihout, but gives an opportunity to think if you're willing to