Found this and thought it would be worth to share!
What does this core edit do?
When you have gm tag activated (.gm on), your name will have a <GM> prefix before your name.
First browse to src\server\game\Handlers, and open MiscHandler.cpp.
Go to line 305, and try to find something like this:
And replace it with:
std::string pname = target->GetName();
When everything is done:
pname = "|cffff0000<GM>|r";
pname = itr->second->GetName();
I found this from another forum that's why i'm only posting his profile page.