As the Gekirangers became apprentices to different Kensei, they each achieved a special skill, which could be applied to their robot. Ran (GekiYellow) gained the Elephant, Retsu (GekiBlue) gained the Bat, allowing the robot to fly, and Jan (GekiRed) gained a Shark.
The first robot got replaced by an even more powerful robot "GekiFire". Once Jan, Ran, and Retsu achieved Kageki (boiling extreme Ki) they where able to summon the Gorilla, Penguin, and Gazelle robots, based after the Kensei helped them achieve Kageki.
When GekiFire stepped in as the Ranger's first robot to use, their original robot was able to be duplicated by GekiViolet.
In the Gekiranger movie, Rio and Mele joined together with the Gekirangers. This happened a few more times during the series, but not often.
The final robot was GekiChopper's SaiDaiOh. This robot actually contained the spirit of Juken's creator, Brusa Ee. So SaiDaiOh is quite revered, being refereed to as Juken's god.
The robot battles of Gekirangers was simply nothing short of amazing. This is the first Sentai series I watched all the way through, and absolutely nothing about the series disappointed. The robot battles, especially, where amazing. The special effects went into over-drive, showing the robots fighting in a flooded out city, flying through the air, or to the bottom of the ocean. And all while keeping the Martial Arts aspect of the series very relevant, even to these giant behemoths.
This image is labeled as coming from www.thai-toku.com