Re the C-17A move on the 24th,
Negative, the 'G' is the first letter of the ICAO location indicator, i.e. GMME an in Morocco.
ZZ177 departed Brize on the 23rd mid-morning as RRR6882 ( refer to the BZZ spotter's blog)
I first 'saw' this flight when it was in the 'teardrop' inbound Northolt on ADS-B , used when flights come off Compton or Biggin. I'm sure it will become known where this Globemaster came from in time....