This is a "Wild Mass Guess" entry, where we pull out all the sanity stops on theorizing. The regular entry on this topic is elsewhere. Please see this programme note.
Grand Unifying Guesses Half-Life aka: Half-Life
Once the Combine has been defeated, G-Man will hire Gordon Freeman out to other universes. Gordon's future jobs and identities include:
The Security Officer from Marathon, who has an aptitude for mindless carnage, no voice and no problem following the orders of a higher being without question. When Durandal merged with Thoth he became aware of the multiverse and the G-man, and hired Gordon's services. After fitting him with a set of Jjaro cybernetic implants, Durandal-Thoth sent Gordon back in time to help Durandal's past self on the Marathon. The G-man lost track of Gordon for a while when the W'rkncacnter escaped and the doctor used his Jjaro implants to dimension-jump, but found him again when Durandal-Thoth released him from its service at the end of Infinity.
The Point Man from First Encounter Assault Recon. Alma is actually G-man's daughter, who was given Gordon to play with to keep her out of trouble while her dad went to an important meeting. She granted Dr. Freeman latent psychic powers, which combined with his Jjaro cybernetics to give him superhuman reflexes. G-man was quite impressed with the mess his daughter managed to make in such a short time, and plans to take her to his agency's next Bring-Your-Daughter-To-Work Day.