Exiftool's QuickTime Tags page lists these (Note: This is not an authoritative source, but still useful):

'gshh'  GoogleHostHeader     string  
'gspm'  GooglePingMessage    string  
'gspu'  GooglePingURL        string  
'gssd'  GoogleSourceData     string  
'gsst'  GoogleStartTime      string  
'gstd'  GoogleTrackDuration  string  

Cheers.