Hewat said Brian To'o's failure to chase Sandon Smith stood out: "Sandon Smith was gassed."
Gelling said the defeat was "symbolic of this as like the end of the Penrith dynasty."
The Knights' victory earned them their first grand final in 25 years, against the Sydney Roosters on Sunday, October 4.
Penrith will miss the finals for a second straight year after winning four consecutive titles from 2021 to 2024.
Gelling cited roster departures and contract discussions, and said Penrith needed Cleary with Jarome Luai and Stephen Crichton absent.
Cleary started the preliminary final on the bench with a broken right foot, entered in the 26th minute, then suffered a compound fracture in his left leg during a tackle.
Gelling questioned playing him: "The only reason you play him is if, 'okay, we really need him.'"
Gelling expected a late surge, but Penrith's shift toward To'o misfired; Smith intercepted the pass and ran away to seal Newcastle's win.