The real Jay Garrick says that he's from "Earth-3," while during the Supergirl crossover, Barry describes Earth-2 as the "one where we're all evil." This is something of a reversal of the comic books, where Earth-2 is the home of Jay and an elderly Justice Society of America, and Earth-3 is the home of the evil counterparts of Barry and other members of the Justice League.
The Time Wraiths were inspired by The Reapers from Doctor Who (2005). Producer Andrew Kreisberg is a known Doctor Who fan and frequently hires actors who have been in the British show.
At 29:00 Zoom says to Barry "Run, Barry. Run.". This, together with "Sit, Barry. Sit." are recurring memes throughout the series.
Henry Allen was born in 1955.
This episode begins one second after the previous episode ends.