ConsoleCommandSender console = Bukkit.getServer().getConsoleSender();
String commandShowDeathMessages = "gamerule showDeathMessages false";
Bukkit.dispatchCommand(console, commandShowDeathMessages);

// But don't forget to write them by yourself in PlayerDeathEvent

// PS: btw if you plan to run that command on plugin onEnable, 
//you shouhld use it in BukkitRunnable.runTaskLater()