Since I believe both of these final 2015 weeks were not posted anywhere else on the internet that I could find (and not re-posted at GameShowFollies, which is where I check first), I asked for them (Source: Douglas Pucci/ProgrammingInsider).
December 14-20: 349,000 total viewers in primetime/302,000 total viewers for total day
December 21-27: 416,000 total viewers in primetime/330,000 total viewers for total day
Did Hellevator make GSN averages crash?
Notice the uptick from the week of December 14th to December 21st. The week of December 14th was the last week with Hellevator on the schedule. Could very low-rated Hellevator leaving the schedule cause GSN averages to tick up that much?
If we only had these two weeks of ratings to explore with, I would say these two weeks could just be too small of a sampling to judge. The week of December 21-27 saw GSN having the highest primetime average since October 19-25, 2015 (the week Hellevator premiered and would have its highest rated episode in total viewers in the series). The week of December 21-27 saw GSN having the highest rated total day average since the same week, October 19-25, 2015.
Hellevator's ultimate crash really did affect GSN ratings that badly. The week Hellevator left the schedule is the week GSN averages ticked up to what they were before Hellevator's crash.
GSN is still looking into a Hellevator renewal. If it does get renewed, GSN executives are not taking total viewers into account and not too much of an emphasis on 18-49's either. GSN execs are just looking into the median age of all eight Hellevator new runs, which was well under 50 years old.