I think we can all agree with Breen being the administrator of Black Mesa, right? So, as the administrator, he should be aware of all experiments that Black Mesa does, and all of Black Mesa’s devices. Right?
So, if Black Mesa created the Gravitygun, why didn’t Breen know what it was up in the citadel when the Combine Elite showed it to him?
“Oh, what’s this? Put it over there.”
And I can accept that the HL2 gravitygun was built by combine technology, because if Aperture builded it, then Black Mesa might have stolen the blueprints of the gravitygun, but never finished it, maybe they got hold of the blueprints right before the Black Mesa incident, Kleiner or Eli brought them out of BM and later made another one out of combine tech based on Aperture’s blueprints. I don’t think that Breen would have any intrest of blueprints anyways.
About the test chambers; I can bet that we didn’t see everything of the old Aperture’s test chambers anyways, you saw more of them in multiplayer then you saw in singleplayer. There might have been more of them. And maybe they had some small chambers on the Borealis for gravitygun testing.
This brings me to another theory: The combine finds blueprints of the gravitygun, Alyx, Eli or Kleiner manages to steal the blueprints from the combine.
Kleiner having blueprints just like these in his lab just makes my theory stronger.