I have also had exactly the same error this morning. I'm running AVG 2012(free version), TMG v7.0, Win7 64bit.
My tmgss.exe date is 26Dec2007 size is 673,040
I checked a backup computer of mine that is running the same configuration and the second I accessed the TMG folder with this program I got the AVG virus warning.
I suspect the AVG is wrong.