First of all, this is simply a beautifully animated series, both in characters and background - colorful and simply pleasing to watch. You could view this simply for that alone, as long as they didn't give you a terrible story - and they didn't! The story and drama are not overwhelming, certainly, but there is pretty much only one story arc, and the creators stay on the mark for the whole series. You could see from the extra characters & situations that were brought into the series that they were no doubt tempted to pursue other story lines - like other anime do & never quite resolve - but in the end they chose not to and stayed faithful to the singular plot line that they began with.
The main concept finds present day humans - sometime in the future of course - the product of past scientific tinkering, along with a dying sun, which first created a frozen planet, and then when things warmed up, a water planet. Everyone lives on gigantic ships which band together in fleets, making de facto cities - this particular fleet being "Gargantia." The "verdurous planet" is of course Earth, with a few tweaks and changes. The humans in this world must eke out their existence on board these ships to create a lifestyle & mini-civilization that works for everyone. We do have some occasionally-bad guy pirates - there will always be those - who will come by to wreak mild havoc now and then, but that's life on the verdurous planet. The people of this world are simple, ultra naive, and occasionally stupid, but most of them seem to have good hearts, which they believe is their strength. Society's fundamental morality now has become the sanctity of life, and one of the ethics of the story is what it means to be human, juxtaposed against the many machines they use, and the sea creatures which they will only kill for food.
Into this basically pacifistic mix comes a young soldier, Ledo, who has been thrown back in time through a wormhole, and winds up on this water planet. Everything is strange to him, including the language. He pilots an advanced computerized robot, which he calls Chamber, and which is for all intents and purposes an A.I. being, except it takes its orders from Ensign Ledo. Ledo must learn to adapt in the new world, for where he comes from, humans only have worth as warriors with no free time or private lives whatsoever. He finds everything in this civilization random, disorganized, and purposeless. He doesn't understand saving the weak, for in his reality, they are valueless and "culled out." Only the strong survive and have worth there, and that worth is judged by their battle expertise. They are at war with alien beings which resemble giant squid type things, and killing them is Ledo's and all his people's entire existence. Here, on the verdurous planet, he must come to understand relationships, helping the masses and the weak, getting a job that is not killing, and finding his heart. However, he discovers giant squid type creatures here on Earth that resemble the aliens he fought in space in his prior life; after his AI robot does a DNA analysis, he finds that the Earth squid and the space aliens he fought are the same creature, which provides a mystery element to this series. Ledo is sworn to killing all these creatures, which puts him at odds with the pacifistic Earth people. That conflict must ultimately be resolved.
Altogether, this is a simple and straightforward story which has many rich elements in it, not only visually, but in its story. The characters are diverse, and mostly normal, with some young females that are shown as pretty sexy in their bathing suits & bikini garb, just so one doesn't forget that beauty is still important even on the verdurous planet, but the creators don't make a big thing of it, it's just there to be appreciated; that's all there is in the way of fan service. The series and story do not overwhelm with many elements and sideline plots, rather "Gargantia ..." tells the one it started with beautifully, interestingly, and adeptly. I highly recommend it as a very pleasant and relaxing bit of entertainment.
