I am surprised I couldn't find this post elsewhere.
Hence, let me be the one to do it.
Here's how you reach the room:
1. Ofcourse it starts at the other end of C3 map.
2. Once you enter Grinta Heath from there, keep entering the doors located at following coordinates:
125,99
99,121
126,218
125,153
216,123
125,154
126,216
126,152
217,124
124,152
126,214
126,151
33,123
126,154
And at the end, Voila! G-Man! (if someone hasn't killed it already )
Hope it helps! Thanks a lot Socara for showing the path.
-IGN Uhlala