[Bacardi]

what if you remove double quotes \" \" ?

*update
Not work in this game mode (custom/custom)

Spoiler

Just make sure:
...csgo/gamemodes_a.txt
-gamemodes_serverfile gamemodes_a.txt

and reboot or launch server.

Again, you should see "exec" files using command on server console (not client):

] rcon sv_game_mode_convars
exec {
  exec gamemode_myfile.cfg
  exec gamemode_myfile_server.cfg
}
GameTypes: dumping convars for bot difficulty (none:0):convars {
  bot_difficulty int( 0 = 0x0 )
  bot_dont_shoot int( 1 = 0x1 )
  bot_quota int( 0 = 0x0 )
}

*edit
Testing again, there could be something fishy going on. Could it be it not work ? I need test again.