What is the influence of the guitar cable (specially its length) on a guitar sound quality?

The guitar cable has got a parasitic capacitance. Capacitance depends on the length of cable and its structure. Even higher class cables have got at 6 meter length so big capacitance that it affects guitar signal (reduction of high frequencies).
This influence depends on "guitar electronics" (should not be considered for active "guitar electronics") and is the most significant for guitars equipped with 500 kOhm volume control potentiometer. This influence depends on volume knob position (the biggest is in the medium position). For this reason the length of the guitar cable should be limited (suggested length is 3,5 to 4,0 meters). Very short (1,5 m) and higher class cable affects in significant boosting high frequencies.

Using the GSC enables to use short guitar cable (a cable 4 meters long allow to move inside a circle of 6 meters diameter in front of an amp).