Suisei no Gargantia, Cross Ange: Tenshi to Ryuu no Rondo Recommendations
Both series are in war with 'aliens' (Cross Ange > DRAGON's, Suisei > Hideous) and fight against them with robots. The main characters also get in a total different environment then what they are used to and see a whole new world open up to them.
As someone who generally doesn't like mecha anime; these are two mecha anime I actually did enjoy. Also, they have some similarities and contain similar plot twists. Both anime have a main character that gets seperated from their home, and end up in a poor isolated area. The war against the DRAGONs in Cross Ange is also very similar to the war against the Hideaze in Gargantia.
The main difference is that Gargantia contains more slice of life, while Cross Ange contains more ecchi.
Dragons = Hideauze
give it a shot to figure it out