GetThreadSelectorEntry throwing ERROR_NOT_SUPPORTED for x64 App - debugging

Question

I trying to write a very simple app to debug a Win32 64-bit app. My end goal is to get the TIB and PEB of the remote thread, but for some reason the way I did this on 32-bit app is not working for 64-bit ones (Aside from looking at Esp vs Rsp and checking SegFs vs SegGs). The code I'm trying to use is here:
#include <windows.h>
#include <stdio.h>
#include <tlhelp32.h>
int main(int argc, char *argv[]){
LDT_ENTRY entry;
DWORD pid;
HANDLE hThread;
HANDLE hSnapshot;
CONTEXT context;
context.ContextFlags = CONTEXT_CONTROL;
if(argc < 2){
printf("Usage: %s PID\n", argv[0]);
exit(1);
}
pid = atoi(argv[1]);
THREADENTRY32 te32;
te32.dwSize = sizeof(te32);
hSnapshot = CreateToolhelp32Snapshot(TH32CS_SNAPTHREAD,0);
while(Thread32Next(hSnapshot, &te32)){
if(pid == te32.th32OwnerProcessID){
hThread = OpenThread(THREAD_ALL_ACCESS, 0, te32.th32ThreadID);
if(!hThread)
exit(1);
if(SuspendThread(hThread) == (DWORD) -1)
exit(1);
if(!GetThreadContext(hThread, &context))
exit(1);
printf("Rsp = 0x%x\n", context.Rsp);
if(!GetThreadSelectorEntry(hThread, context.SegGs, &entry)){
LPSTR buff = NULL;
FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL,
GetLastError(), MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&buff, 0, NULL);
printf("Error: %s\n", buff); //ERROR_NOT_SUPPORTED 50 0x32 The request is not supported.
LocalFree(buff);
exit(1);
}
}
}
CloseHandle(hSnapshot);
return 0;
}
but it's always throwing an error at "GetThreadSelectorEntry". The error code that's thrown is ERROR_NOT_SUPPORTED: The request is not supported.
I am not able to understand why it's not supported. Does anyone know why?
[EDIT]
Okay GetThreadSelectorEntry is not available to x86_64 processes, does anyone know how I can get the TIB/PEB addresses of a remote process?