The coupling capacitor CG blocks the high DC voltage at the plate of the driving tube while allowing audio signals to pass:
Unless bass attenuation is desired the capacitor needs to be large enough to induce very little signal attenuation at bass frequencies. (82Hz is the lowest note on a guitar with standard tuning.) It shouldn't be too large, however, or unwanted effects like blocking and motorboating may ensue.
The calculator computes the gain at various frequencies based on the capacitor value and volume control setting. It assumes that attenuation due to the grid-stopper resistor RGS, if used, is only significant above audio frequencies, which is quite valid because it is used to prevent radio-frequency oscillation.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).