Choujikuu Seiki Orguss

Guy with mech teams up with foreigners and forms a bond. Classic animation styled animes. Each is similar mecha. Adventure/episodic feel. Nature 

Both are set in space and have a lot of mecha action. Both titles have a pilot is thrown into an alien world as a result of some kind of space phenomenon. However Orguss is way older than Gargantia and the animation won't look as modern. If you don't mind watching older anime, if you like one then you may enjoy the other as well.  

Macross and Orguss share virtually the same key staff, from director, to character and mechanical designers, to composer, to writers, etc. And above all they have a similar vibe with some serious drama here and there tossed in with a little fun comedy at times, and you'd almost believe they're set in the same universe. All in all Orguss feels like it makes a better unofficial sequel to Macross than any of the official sequels Macross ever got.