So what happened in the first place anyway?
Probability: What if Zordon in human form taught Gosei in human form years ago before MMPR but after Rita came out of the dome, Zordon left Gosei in the Command Center(Megaforce).
When Gosei saw that Zordon was sacrifed to protect the earth, Gosei knew that it was to protect the planet. So now, Gosei became earth's guardian replacing his mentor Zordon. Gosei had time to create the zords, and Robo Knight, and when Gosei was busy, he would send Tensou to a playdate with Alpha (which in Power Morphicon we saw that he ran out of batteries).
This is just a probability, post comments to share your thoughts about this.
This is just my theory, but here it goes....
I'm going to guess that Gosei was just a normal guy (who lived hundreds of years ago.). Zordon took him on as a student in case anything ever happened to him so that he could take over his duty's.
The reason he's a giant robot head is probably one of these two: One, at one point he was injured from a fight and was dyeing. The only way he could save himself was to transfer his mind into a giant computer, head... Thing. Or (much more likely...) two: Saban just wanted to have it by like MMPR and made him a head.