The former, I say.
I was thinking of something roughly cell phone sized. I know that's a bit of a stretch, but it's all been educated guesses thus far. Remember though that G-man and his have had way longer to screw with those crystals with way more advanced means than even the Black Mesa...
Actually, it stands to reason that after the events of 1, the state of Earth has no bearing on Gster's teleporting prowess. Remember, it's as likely as anything that he essentially owns Xen at this point.
As for the Combine catching on, that's up for debate. Who's to say they would notice at all?
Just a thought, but dig this:
1. After extensive dimensional screwing around, Black Mesa finds Xen, sends teams, nabs animals, crystals, etc.
2. The government finds out, as they sometimes do, and appoints sends G-man, a fellow of some considerable rank is some agency we're not allowed to...
Or he could be in such a state so as to not know whether the stings are actually there, whether his spine is still attached, etc.
But yeah, tone down the words just a tad, and you may want to explain 'aegis'.
EDIT: Like the cruiser?
In fact you've just pointed out what I think is the biggest whole in that theory.
Combine contact Breen. Not a fact, by the way, but now I'm just splitting hairs.
Firstly, how? If they can contact him, why can't they teleport to him? Why do they need him?
And Laivasse, what does all that mean?
Hey, any opposing theory that does not include the phrases:
"gman am alien cuz he screws with time"
"CROMBINEPWNXENJUSTLOOKATASSJETANDNILINANTHSEZSLAVES!!!111"
Or of course,
"Laidlaw said the website was the real deal"
Is just fine with me.
Hm. I still have difficulty believeing that they could find a guy, any guy, contact him, have him be on board with the humanity sans instinct affair, then predict that some scientist will launch the right sattelite and kill the right anther to get the storms working, and then invade and go...