Generally it would have a gain of 1 (no gain at all) for it's passband, and attenuate below that, based on so many dB's per octave, depending on the slope of the filter.
An equal-value components (R and C) two-pole Sallen and Key Butterworth filter needs a gain of about 1.6. Google for Sallen and Key Filter and you'll find a list of component ratios vs gain required.
The gain determines the damping of the filter. Too much gain gives a peaked response near its cutoff frequency, like I use for my stereo's and clock radio's bass-boost circuits. Way too much gain and you have an oscillator.