I recently heard someone say that this show was planned to be two cours. That felt truer after episode 4, but by the end of the series the broader questions were answered, and I did not think that it needed a sequel, especially around the time when there were rumors and speculations for a second tv season. A sequel materialized in the form of two 50 minutes plus OVA's, which I also own and watched them alongside the tv series and earlier short OVA's.
After watching around 7 hours of content Gargantia remains one of my favorite animes of all times. This show can be categorized as science fiction, but I am more attracted to the main characters, the sense of community, some mystery and of course the very nice visuals. A proper second season has not materialized, but I am pleased with what has been made. I would love to see more anime content, but OVA's are more realistic than another tv series.