Correction to that list. The Medusa is probably already in service and probably had been for a while. The Athena was supposed to be the GTA's oldest bomber. It was still fast but wasn't big enough for the new classes of bombs being used.
So the GTA fleet would be more like this:
GTC Leviathan (produced alongside the Fenris for many years)
GTF Angel (in the process of being decommissioned or sent to second line squadrons)
GTF Valkyrie (or "super Angel" if you use US military style naming, upgraded, in op eval)
GTB Athena (old but still frontline)
GTB Medusa (newish to the war I suspect)
And of course we know that the Hercules and Ursa were in the prototype stages and rushed into production.