On the G-Man Dialogue:

He speaks in a sort of non-periodic stutter, with nice and long s's and what seems to be a struggle to breathe. He does seem to hesitate the first time he mentions his "employers" to Gordon. However, this does not seem to be directly written by Marc Laidlaw, so this is not to be taken as canon. Every once in a while, he just stops speaking, either to emphasize words, to choose his next words, or because he has trouble speaking. His speech does become closer to the average English speaker as the series goes on.

G-Man speaks using a lot of business-related metaphors; for instance: "employers", "appraisal", "investments", "hire", and "value"; the train motif also appears in the ending of Half-Life: 2. He also references time and consequences a great deal. In Half-Life: Alyx, he deliberately points to what Alyx has done and says "As a consequence of your action: this entity will continue, and this entity will not."

Now, the content of his speeches. In Half-Life, he explains to Gordon what has happened and offers him a job; he also gives Gordon the "illusion of free choice", as it is referred to in Half-Life: 2. 

In Opposing Force (which I do consider canon), he does something similar to the above, but he says that those who beat the odds somewhat remind him of himself. This could be G-Man making an "investment" of sorts in Shephard. 

In Half-Life: 2, he does something similar to Half-Life, but he says that "The right man in the wrong place can make all the difference in the world". The meaning of this is unclear and it is said in a way that seems like a warning of sorts to Freeman. At the end of the game, he completes Alyx's sentence and tells Gordon that there are other offers for his services, and that these are "extraordinary times" that even he can't ignore. He says that he's "really not at liberty to say", confirming that he either has people above him, or that such information would destroy his plan.

In the Episodes (I'm grouping them together because Ep1 doesn't offer much), G-Man says that "We'll see about that." in reference to the Vortigaunts taking Gordon. This implies that there is nothing he can do about the Vortigaunts, but that justice, per se, will be served eventually. Later, in the mines, G-Man confirms that he has little control over the Vortigaunts, and it seems that he appears to guilt-trip Gordon by using what Gordon did in Black Mesa against him. He reveals that he is the reason Alyx made it out of Black Mesa, and that he was told that she was practically useless, and that he remains confident in his initial appraisal of her. He implies that Gordon is one of his many investments, and that, because he doesn't usually squander such things, Gordon is an exception. He orders Gordon to get her to the White Forest base, and tells him that he has agreed to operate under certain restrictions. Finally, he tells Alyx to relay a message to Eli: "Prepare for unforeseen consequences."

In Half-Life: Alyx, he commends Alyx for her work and says that Gordon would require far less to be imprisoned than what the G-Man had. He dismisses Alyx's query regarding who he is and instead says that he can offer her something in exchange for coming all this way. He then explains that his employers believe that the fate of their worlds is inflexible and that they authorize him to nudge things in a particular direction, occasionally. Then, he asks what Alyx wants nudged; when told that Alyx wants the Combine off Earth, he shows her what happens at the end of Episode 2. He then commands Alyx to release her father; when she does, he informs her of how she has proven her value, of Gordon's defiance, and his struggle to find a suitable replacement for Gordon. He also informs her that his search for this replacement is now over, implying that Alyx is the replacement.

Over the course of the series, G-Man appears to become more and more discomposed. He starts as someone (or something) who would explain things succinctly and would calmly dispose of unwanted material. In Episode 1, he seems angry as the Vortigaunts take Gordon away. Episode 2 has G-Man seeming passive-aggressive with Gordon. Finally, in Half-Life: Alyx, G-Man appears to make a very rash decision in allowing Alyx to change the course of history; time travel, according to Portal 2, is a very finnicky thing.

Though we do not know what the G-Man is, we can make assumptions based on our knowledge. I will share my personal favorite theory. I believe that G-Man is a fully developed shu'ulathoi who is seeking to free his brethren from Combine captivity. As we know, the Combine Advisors are the larvae stage of shu'ulathoi and even then they are very powerful. At the end of Half-Life: Alyx, he even seems sad that he had to kill one of the Advisors for Eli to live.  

And that is all the dialogue written for G-Man. I'm writing this because I would love to see G-Man written closer to his canonical appearances in fan-made works. I would also love to write a G-Man speech for a fan-made work myself, as this character seems very fun to write for. That will be all from me, goodbye.

Edit: I forgot I included Opfor in here. I made the decision to forego HL:A because I was tired by the end of writing this. I am aware that Opfor was not written by Marc Laidlaw, I had just included it in a draft that was far too ambitious and which had to be scaled back. Funnily enough, that is the same reason there are references to HL:A within this. I kept both of them because I find that dialogue very interesting. I also made an amendment to the Ep2 section: I added the implication G-Man makes regarding Gordon being an exception. I have also made a grammatical correction: previously, I referred to White Forest as "White Forrest".

Edit 2: I added the three paragraphs after the analysis of the Episodes. I also changed other parts, including the title, to better accommodate that portion. They are as follows:
     Removed "I will not be including a full analysis of Half-Life: Alyx, for Marc Laidlaw was only consulted for that game. There will only be references to it."
     Changed "And that is all the dialogue Marc Laidlaw wrote for G-man." to "And that is all the dialogue written for G-Man."
    Changed the title from "On the G-Man Dialogue Written by Marc Laidlaw:" to "On the G-Man Dialogue:"

Edit 3: Another interesting thing, Breen alludes to the G-Man in his final moments. He says that Gordon is a "fine pawn for those who control him". "Those who control him" very clearly alludes to the G-Man in that Gordon is "employed" by the G-Man. Breen queries Gordon regarding whether he knew his "contract was open to the highest bidder". Breen's final words, "You need me!", while they could be a bluff or bargain for his life, suggest that Breen could be instrumental in fighting the G-Man (we know the G-Man is a villain because of the way he was described in the HL1 25th Anniversary Documentary).