The opamp in both circuits is just a voltage follower with a gain of 1, so it is irrelevant for the purpose of calculating gain.
The left circuit is a simple R-C high pass filter, and the right circuit a simple R-C low pass filter. The gain of each of these is 1 well into the passband. Well into the stopband, the gain will decrease 6 dB/octave or 20 dB/decade of frequency.
The rolloff point of either type of filter is when the capacitor's impedance magnitude equals the resistance. The equation for the frequency is:
F = 1 / (2 π R C)
When R is in Ohms and C in Farads, then F is in Hertz. In your case you have 100 nF and 3.3 kΩ, so the rolloff frequency of each filter is about 480 Hz. At that frequency, the filter will attenuate by a factor of the square root of 2, or will have have a gain of -3 dB. The filter gain as a function of frequency varies smoothly, but after a octave or two in either direction it approaches 20 dB/decade down from the rolloff frequency on one side and unity gain on the other.
The left filter is a high pass, so for frequencies above 480 Hz it will approach unity gain as the frequency gets higher. After about 1 kHz the gain will be close enough to 1 for most purposes, certainly for any normal audio application. Well below 480 Hz, it will assymptotically approach attenuating by the ratio of 480 Hz to the actual frequency. For example, at 100 Hz it will attenuate close to 4.8 times, or the gain will be close to -14 dB.
The low pass filter on the right works the same way flipped in frequency around the 480 Hz rolloff value. At 100 Hz it's gain will be nearly 1, and at 3 kHz it will attenuate close to 3 kHz / 480 Hz = 6.25 times, for a gain of -16 dB.