Odin just wanted a son that wasn't 100% Asgardian, he wanted his first born to be more.
Odin's plan or desire to produce a half-Asgardian makes little sense. Odin himself has mixed lineage, and the very origin of the gods is adjacent to the Jotunns. The gods are a hodgepodge of Jotunn, Vanir and Aesir. As such, even Loki has a rightful claim on the name.
In addition, Gaea's genetic 'make up' is not really that of the Earth. She may have had a natural affinity for the Earth, but she developed her Mother Earth nature and powers by merging her life-force with that of all life on Earth after the Elder Gods purge. Set had previously accrued power by absorbing other Elder Gods (and co-opting reptilian life), and Chthon delved into the dark arts.
Odin's plan makes more sense if all of Asgard is connected to the Odin-Power, and Odin wanted an heir with non-Odin-Power value. There is wording in the origin relating to that... but it was crafted at a time when Odin was the embodiment of a previous generation of Asgardians.
Still think that Odin wanted to make Thor that way in order to have Him just as power on or out of Asgard, to deal with returning Celestial Host!