Giriel beat me with the answer, ok, 2nd

You can't change it with a hex editor.

You could recompile the server (etded.x86) with a bigger MAX_GAMESTATE_CHARS, to avoid the 'MAX_GAMESTATE_CHARS exceeded' message, but I see a big problem here:

In the mod, you have (in Cg_local.h)

typedef struct {
gameState_t gameState; // gamestate from server
[... other members ...]
} cgs_t;

gameState_t size depends on MAX_GAMESTATE_CHARS, so you will have a bigger one in the server, and a smaller in the mod.
When it's retrieved by the client via trap_GetGameState, it will overflow over the memory area of the other members of that struct, causing all sorts of bugs and crashes.

So if I didn't overlook anything, I think there is no easy way out, you'd better find the cause of that problem (happens only with some maps?)