The Theta Gate was an interdimensional teleportation device used by Blackout to bring his infinite army into the Matoran Universe. It is unknown whether Blackout found the Gate or built it.
Zev has been working on developing his own similar device called the 'Betta Gate'... However, he can only make hybrids of Toa and Siamese fighting fish come out of it, and if he summons more than one hybrid at a time, then they just fight each other to death.
Seriously. Only Zev could screw up that badly.