Chogokin (超合金) is Japanese for "Super Alloy" and is a fictitious material first appeared in Go Nagai's Mazinger Z comic. It was later adopted by the Popy toy company in early 1970s as a new line of die-casted metal robot and character toys sold in Japan. While Bandai, the parent company of Popy toy owns the trademark to the brand, the term is generally used to describe any Japanese diecast robot toy made from zinc alloy.
Chogokin toys were generally produced in ST and or DX sizes. ST meaning standard and usually in the range of 5" tall. The DX versions were deluxe and usually much larger. In addition to the larger size, the DX versions usually came with more features such as separating parts and more complex weaponry, usually with launching projectiles. (Wikipedia)
Popy began the Chogokin series by giving each toy a unique number, such as GA-01. This practice continues today with all of Bandai's Chogokin lines such as Super Imaginative Chogokin and Soul of Chogokin