Reclusive Artist: Tom Siddell is known for being very private about his personal life. All we know for sure about him is that he lives in Birmingham, used to have a job as a graphic artist at an undisclosed video game company, and is pretty good friends with Magnolia Porter.
Schedule Slip: Not the webcomic itself — Tom has maintained a 30-page Strip Buffer since day one — but both published volumes didn't hit shelves until months after they were scheduled to come out. The delay of Orientation was bad enough that Tom thought Archaia was defaulting on the publishing contract.
Series Hiatus: Several times, each one lasting, at least, two weeks, and coinciding with the end of each book.
The first was after the end of Chapter 14: The Fangs of Summertime. In the timespan between this chapter and Chapter 15: Red Returns, bonus material and guest comics filled that void.
The second hiatus occured after Chapter 22: Ties, and this time it was the omakeCity Face.
The third hiatus happened after Chapter 31: Fire Spike, and, like the second time, featured City Face 2.
The fourth hiatus happened after Chapter 41: Changes, and featured in-universe cartoons about City Face, ostensibly drawn by some of the comic's main characters.
Chapter 40 is essentially a long, drawn-out series of scenes continuously depicting Jones throughout the ages, getting progressively longer and longer, without actually explaining what Jones is, culminating with Jones being present at the formation of the planet. At the end, Jones finally tells Annie that she doesn't even know what she is, with Tom's comment being, well....
What Could Have Been: According to Tom's Formspring, Zimmy (a.k.a. Zeta) and Gamma were originally to star in a comic idea known as Alphabet Soup (hence their names). It was also mentioned that Zimmy was around before even that and used to look quite different. Also, Tom considered naming Annie Imogen before his idea for Gunnerkrigg Court consolidated in his mind.
He's mentioned later on that he might still want to do the comic about Zimmy and Gamma as a spin-off prequel of Gunnerkrigg Court, some day.
After receiving a considerable amount of steady donations of Patreon, he has promised to make this comic reality in the near future.
Word of God: Tom Siddell frequently posts on the forum or Shout Box to clear up confusion or give background info. There's an article in the GC Wiki for an index of such information. He also has a Formspring account where he answers questions about the story, and a Twitter account. Somebody built a handy search engine of the 3000+ questions he answered, which can be found here.