The reference point
Screen gain compares reflected light in a reference direction with a standardized diffuse white surface commonly treated as gain 1.0. A nominal 0.85 value is therefore lower than the reference in that measurement direction, but it says nothing by itself about perceived contrast in a lit room.
Why lower gain can still look better
Black level in projection is strongly affected by ambient light. A directional ALR screen can reduce room-light contamination and make dark scenes look more convincing even if its nominal gain is below 1.0. That is why gain and ALR must be read together.
0.85 versus 0.6 in this product family
The PET Crystal Classic is published at 0.85 gain, while the NothingProjector Black Series floor-rising screen is published at 0.6. The Classic therefore occupies a different balance point: higher nominal reflected brightness, with a lower published ALR percentage than the Black Series.
Practical takeaway
Choose a screen only after deciding image size. A projector that is comfortably bright at 100 inches may have less brightness headroom at 120 inches because the same light is spread over a larger area. Screen gain is one part of that system-level calculation.