By strict definition, a low pass gate (LPG) is a low pass filter whose cutoff frequency goes down into the subsonic range as its control voltage goes towards 0 volts, resulting in the input signal being filtered almost into silence. Some replicate this by combining a low pass filter and a voltage controlled amplifier into the same module, with both following the same control voltage. In either case, as an input envelope falls from a high level to 0 volts, the output gets duller (higher harmonics are filtered more) as it falls to silence. This mimics the way many natural sounds work.
In practice, LPGs are given near magical, mythical status, based on their origination in the Buchla 200 series back in the stone age good old days. They tend to use gentle low pass filters in their design (often only 6 dB/octave), and are controlled by vactrols: LEDs coupled with light-sensitive resistors, which are prized for their quirky behavior such as a nonlinear response and delays in how fast they rise to minimum resistance and decay back down to maximum resistance. Since the low pass filter cannot go all the way down to negative infinity, quite often a little sound leaks through even when they are “off.”
LPGs are a mainstay of the West Coast Synthesis approach of starting with a simple oscillator waveform, modulating it or otherwise altering its shape to add harmonics, and then gently filtering them with a low pass gate. However, they are equally at home in sequenced or arpeggiated East Coast patches.