Code:
EGCResults SteamWorks_GCRetrieveMessage(int punMsgType, const char[] pubDest, int cubDest, int pcubMsgSize)
EGCResults SteamWorks_GCSendMessage(int unMsgType, const char[] pubData, int cubData)
These two seem to be slightly broken, "SteamWorks_GCRetrieveMessage" doesn't work at all and "SteamWorks_GCSendMessage" only works if there is no "SteamWorks_GCRetrieveMessage" forward.
Looking at the source on Github it seems to be the fault of lines
132 and 134.
should be
Attempting to fix and compile this myself made the entire extension not do anything anymore while apparently loading successfully. Maybe something related to what SM/MM/SteamWorks version I compile it against though I tried a few different combinations and none worked.