Half-Life 2 Other Ending (Kill G-Man)
Please note that this is not Half-Life 2, but Garry's Mod.
This is not possible to do in the original game.
I have some reasons on why this ending could happen:
The G-Man captures Alyx in that box because Alyx was concerned on what Breen said about his contract. "Did you realize your contract was open to the highest bidder?"
Gordon simply kills G-Man by exploding a barrel because G-Man captured Alyx.
As G-Man explodes, Father Grigori is heard saying "Careful, brother!" in the doorway. Maybe he retrieved the vortigaunts power to enter the G-Man's realm.