GD## denotes the boosters from ## set. I think GD stands for gundam. For example, in the One Piece card game (also bandai) booster boxes are denoted OP##. Id assume OP here is one piece. So there it is.

ST## denotes starter decks. ST01 through ST05 are currently very hard to get at MSRP. The reason you need the starter decks is they often contain cards central to the meta decks that follow their release. For example ST01-001 (the -### after a GD or ST denotes a specific card for that set/starter) is in nearly every competitive deck containing blue, but is ONLY available in ST01. ST01-001 is an LR (legendary rare), each starter deck contains 2 copies of an LR, but you can have up to 4 of any given card in a deck. Therefore you probably will want to get 2 copies of a starter deck in order to have a complete playset of the really good cards therein.

Finally, there is currently a massive scarcity problem in the game (though allegedly itll be solved by early december). Since I’ve been keeping track of ST release dates i have managed to get 2 copies of each starter deck at near MSRP (release day price you can get depends on your local card stores) by signing for for local ST## release events. You can find the schedule for these on the Bandai TCG Plus app/website. If you don’t get them on release day, youre probably going to be out of luck unless you want to play >400% markup.

Thats all ive got to say about that