The actor Johnny Lewis who plays Gabriel unfortunately, like his character, suffered from mental illness. He died at 28 after murdering his landlady and her cat. His death was ruled accidental.
Jor-El is shown with a physical form for the second time. Previously he made a single appearance when Clark Kent recollected Jor-El visiting Smallville as a youth.
Originally, Martha's line in regards to Clark and Lana ("Please tell me you were at least safe") was supposed to be spoken by Jonathan, but John Schneider objected to the scene as he felt that Jonathan would have been a lot harder on Clark. His reaction on-screen was the compromise between him and the producers.
Gabriel kills the 2 military personnel in the silo that he has taken over, so the military could easily ascertain which silo he was in by just getting it's personnel to check in. The only personnel who wouldn't check in would obviously be the ones who were dead, and the military would know which silo Gabriel was in.
Lionel says the last thing he remembers is arguing with Lex in the library, which occurred in "Commencement".