GameMon64.des is GameGuard anti-cheat... They inject DLLs into every process (including Task Manager, Process Explorer and Process Hacker) and hook multiple system APIs to make their anti-cheat 'invisible' by returning 'fake system data' and fake error codes such as STATUS_ACCESS_DENIED from those hooked functions (which is what you're experiencing).
It's incredibly dumb and the hooks they're using introduce system instability, performance issues and can be bypassed easily (especially on Win10).
GameGuard is not considered 'malicious' and doesn't appear to be doing anything illegal - I don't think its a good idea to discuss the internals of their anti-cheat and the various methods to bypass it without first informing them of these flaws.
Suffice to say if you're using Win10 you can add two function calls into winmain that block their anti-cheat: