The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as "cathode degeneration." This substantially increases gain.
When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but as an open circuit for DC, thereby maintaining DC grid bias. We can introduce treble boost by using a lower capacitor value, one that acts as a short circuit for high frequencies but allows negative feedback to attenuate bass. This is often done for the preamp's bright channel. If the additional gain is unwanted, based on the amplifier's overall gain from the input jack to the power amp, the capacitor can be eliminated entirely.
The calculator computes the gain at various frequencies based on tube, resistor values, and capacitor value. It assumes the coupling capacitor CG is large enough to be considered a short circuit for guitar frequencies. This is almost always the case.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).