It is not possible to set GPIO thresholds.
Input and Output levels are published in Voltage specifications
The I/O ports have two parameters which deal with the input level:
VIL: The maximum low level voltage.
VIH: The minimum high level voltage.
Input levels are typically
Low should be <= 0.9V.
High should be >= 1.6V.
I have done tests on a number of Pi to detect thresholds and to determine hysteresis.
These are basically normal CMOS levels.
The actual threshold is ~ 1.2V.
There is a small variation between individual Pi and the actual levels vary when rising & falling as hysteresis is enabled by default.
When designing circuitry I design for 0.8V to ensure LOW and 2.2V to ensure HIGH. This allows for normal component tolerances.