Where do the GSL league names 'Code S' and 'Code A' come from?

Solution 1:

Because having a B-rank of players sounds bad. A-rank (Code A) players are still excellent, hardly deserving of a paltry, average, "B" grade, so they just shift up the top to something else: S.

As far as what they are, Code S is the big time league, while Code A has been likened to the minor leagues (though still quite an achievement)

More on this S thing from Giant Bomb's "S-Rank" article:

‘S-Rank’ is a term used in games, most commonly awarded at the end of stages with graded missions or side objectives.

Originally created in Japan where anything below grade ‘C’ was considered a failure. The 'S-Rank' [...] If the player is flawless or achieves perfection in something, it qualifies as an S. Many people have wondered what the S stands for… Special? Super? No one knows for sure.