I think those are all just the comics that Valve has already written and released, so I'm a little disappointed that it's not anything new.
And the reason that there is no Gordon Freeman is that Valve's first comic was for TF2, with several more TF2 ones written to promote some upcoming update, before they made "The Sacrifice" for the L4D updates, and the Portal 2 one before Portal 2 came out. Half-Life has been stagnating (as far as we know) all this time, so they haven't made any announcements regarding it. When they do announce HL2: Ep. 3 (Or HL3) I expect a few comics to be there, as well.