Reading that Alyx unknowingly became a sleeper agent to G-man with the task of getting rid of the Borealis (so that humanity can't use it against the combine) with her dad's last wish as a cover kinda blew my mind. That G-man took her away and left Gordon to die on the Borealis, only for Gordon (you) to realize that the suicide mission won't make a dent on the Combine as they are all to powerful is pretty damn dark and fitting for HL, it also makes Dr. Breen comments in HL2 that struggle is essentially futile ring more true.
It's the other way around, the G-Man has always tried to prevent
the Combine, or at least a particular/more dominant faction of the combine, from getting access to Earth's teleportation techs.
1. In HL1 the G-Man allowed Freeman to kill the Nihilanth, while the Combine were closing in on the Nihilanth and were interested in his biological ability to teleport himself and his minions at will.
2. In HL2 the G-Man deployed Freeman and helped prevent the sale of Eli by misdirecting Breen's attention. Gordon, being the bigger threat and prize proved to be the perfect distraction and wasted Breen's time, resources and soldiers (Combine was looking for Eli's vast knowledge on teleportation but the G-Man prevented that by using his pawn, at the time, in Gordon)
3. Now in HL2EP3 the G-Man does the same thing, though this time the tech is levels of magnitude better, he uses Alyx (the daughter of his former subordinate, Eli, and do not forget that while it was the G-Man that got the crystal, it was Eli's call to use the crystal, causing the resonance cascade) to destroy the Borealis, hence preventing the Combine getting access to teleportation tech, the G-Man and Eli had some sort of pact since HL1, part of the pact being Alyx's safety