Got it, that makes sense to me.
Rephrasing to make sure, the GFLow point becomes defined what you get to a point that a compartment is at X% of the M-value. That point can be predicted when you begin you ascent, but can change based of your ascent rate. Once that point is defined it creates the ceiling.
I wouldn't put it that way. The GFLo is a number picked ahead of time, arbitrarily selected, but used (along with GFHi) to generate an ascent plan. The other three variables are depth, mix and bottom time. GFLo is not something that is generated or changed during a dive.
Your ceiling at any give time is defined by a specific overpressure limit. That limit rises in a linear fashion from GFLo to GFHi, over the course of the ascent.