Geminar is an alternate world in another dimension.
During the events of Photon, The Idiot Adventures, it was merely known as "Sandy Planet," and was not technologically advanced, however, most people on the planet were able to use the Ahou Energy in some manner and anti-gravity travel was common. It secretly housed the "Anti-Ahou Generator" which could serve as a check on the out-of-control Ahou Energy in its galaxy or could rewrite the entire universe in the desired form of the operator. After its activation, the planet became a green planet (shown to have identical continents to the Earth).
After 10,000 years it came to be known as Geminar and eventually developed advanced technology on the cusp of interstellar travel capability. Unfortunately a war born from out-of-control out of control Mechanoid creatures wrecked most of civilization and regressed it to one that did not fully understand the advances of the previous civilization. This led to the events of War on Geminar, in which Kenshi Masaki was sent to Geminar, presumably transported to by Tokimi, to battle Gaia, the revived threat of the earlier civilization that could again cause the world's complete destruction.
Some of its countries are the Shtrayu Empire, Shurifon, Havoniwa, Swan, and the Tribble Kingdom. This world has retained its connection to Ahaou energy that still permeates the air of the world.
Time appears to run at a different pace on Geminar than on Earth. Kenshi travels to Geminar around 25 years after his mother, Rea, was first sent to Earth, but on Geminar several thousand years have passed.
Geminar has always seemed to be able to summon people from other worlds, from Earth in particular. These summoned people have often been more powerful in Geminar (one of the reasons Kenshi had to be sent) and have influenced culture. One summoned person from the past seems to have been quite the otaku and instituted a couple slightly perverted customs on Geminar and enshrined them as high culture and tradition. Such as having the the female pilots wearing cat ear headbands, tails, and swimsuits when they are named Holy Knights.
There is a theory that the effect of the Anti-Ahou Generator was the cause of the dimensions splitting.