In a lot of ways, Gaiking feels like Eureka Seven. Boy meets girl, big mecha, boy boards a big ship, girl has mysterious power, etc., except to compare Gaiking to E7 is simply an insult to the latter one. But let's do some comparisons anyways: Theme E7: communication leads to understanding, understanding leads to trust. Gaiking: kill monsters Plot E7: finding out the mysteries surrounding the Corallians Gaiking: kill monstersCharacters
Eureka VS Lulu: Eureka was inspired by Rei from EVA but with genuine character development and actual feelings, the two things make her such a great character (she also has some steels in her!). Lulu likes Daiya, that's it.

Renton VS Daiya: Renton is the most realistic shounen character I've seen. Not only does he receive such a huge amount of development throughout the show, but a lot of things he does, even though making 100% sense, are not entirely right or entirely wrong. Daiya is a good person too. He likes to help out people, he likes to fight off monsters, he works hard, but in terms of a character, he lacks depth.

Mecha Design
Sleek post-EVA-ish mecha VS what looks like when someone took a dump

Animation
Not even a fair comparison between the two.

My final comment is that Gaiking is not worth investing your time in.