This format of numbers and abbreviations (dB/oct = decibels per octave) is often used to refer to the frequency response behavior of a filter. A filter typically has a cutoff or corner frequency it is tuned to. It then reduces (filters) the frequency spectrum of a signal going through it so that its loudness is multiples of 6 decibels weaker for each octave further away you get from the cutoff frequency. A 6dB/octave filter is often referred to as a “one pole” filter (as each pole of a filter’s design results in 6dB of attenuation), and has a relatively weak effect on the signal going through it.
Low Pass Gates (LPGs) typically – but not always – use 1 pole low pass filters, reducing the strength of higher harmonics by 6 decibels for every octave above its cutoff frequency.