On a side note and somewhat related to what Spirit said: Check game modes in OnPluginStart(), not in OnClientPostAdminCheck() because the plugin will check every time someone joins and SM checks the client if he/she has a flag on which may cause lag.
Likewise:
PHP Code:
public void OnPluginStart()
{
char sGameMode[16];
FindConVar("mp_gamemode").GetString(sGameMode, sizeof(sGameMode));
if (StrContains(sGameMode, "versus", false) == -1 && StrContains(sGameMode, "scavenge", false) == -1)
{
SetFailState("[ABM] Plugin Runs In Competitive Modes Only!");
}
// your codes here
}