In the background at the new facility, the Slime Blowers used in Ghostbusters II (1989) can be seen. One of the engineers is spraying another, perhaps to test the effects of the slime.
When Nadeem asks if anyone has a lighter, Ray replies that he stopped smoking in the 90s, whilst this may be a burden to their current predicament, it's a call back to Ray's bad habit, where he can be seen smoking throughout Ghostbusters (1984) and smoking a cigar in Ghostbusters II (1989).
When Gary says he picked out a film for movie night, he holds up a VHS copy of Cannibal Girls (1973). This was the second feature film directed by Ivan Reitman who directed the first two "Ghostbusters" films. He's also listed as a producer on this film which is dedicated to him. The title "Cannibal Girls" can also be seen on a movie theater marquee in both Ghostbusters II (1989) and Ghostbusters: Afterlife (2021).
According to Mckenna Grace, it was somewhat more difficult (at least for her) to work with the surviving original cast compared to Ghostbusters: Afterlife (2021) where the original actors came in late in the film and were only on set for 2 weeks to shoot the climax. In this installment, the original cast were on set for about 90% of the shoot. Grace admitted that while it was fun working with them and hearing their stories, she also felt a little intimidated and worried about screwing up in front of the guys who started this whole thing 40 years ago.
Ghostbusters: Frozen Empire is dedicated to the late Ivan Reitman, the original director of the first two movies who passed away on February 12, 2022.