Gideon says, "Hello Dr. Wells, it has been 1078 days since we last spoke." That date is April 28, 2015, the day "The Trap" premiered, which was the last time she appeared in the present day (as she appeared with the past version of Eobard Thawne in Potential Energy).
While not mentioned by name, the cameo pairing of Kevin Smith and Jason Mewes is an obvious nod to Smith's characters, known to any half-decent K. Smith fan as 'Jay and Silent Bob', right down to how the former talks intensively and the latter does not say a word.
Janet Petty/Null is the fifth case of a genderbent antagonist from the Flash comics, the other four are Brie Larvan/Bug-Eyed Bandit, Rosa Dillon/Top, Mina Chaytan/Black Bison, and Izzy Bowin/Fiddler who were males in the comics
Breacher says he was attacked by someone named Crucifer and his bloodsucking minions of the Tenth Circle, to which Cisco remarks that he was attacked by "vampires", Danny Trejo was in Dusk Till Dawn (1996) and its two sequels which was about vampires.
In the comics, Null is a metahuman enemy of Hawkman.