The GSN-19 was part of the BS-1 Tishina system, which included the GSN-19, a folding stock AKM, a PBS-1 silencer, and subsonic ammunition.

The BS-1 “Tishina” (Silence) grenade launching system was developed for Soviet Spetsnaz.

The idea behind a “noiseless” grenade launcher was to hide the sound and flash of the grenade launch and thus give the shooter (and his comrades) a greater chance to escape after a successful diversion behind enemy lines.

The GSN-19 silent (flashless and noiseless) grenade launcher is unusual in the fact that it is both muzzle and breech-loaded. The standard projectile is a specially designed AP-I grenade with pre-engraved rifling.

To achieve the noise-less and flash-less launch, blank cartridges are discharged into the limited volume breech, which is fitted with a movable telescopic piston at the front. Upon discharge, this piston violently pushes the grenade out of the muzzle of the launcher, and then stops and seals the expanding powder gases inside the rear part of the launcher’s barrel.

It seems similar in concept to the 7.62x42 SP4 ammo but in a grenade launcher form. So here’s a cutaway of the SP4 ammo:

Here’s some presumably Russian dude talking about it on YouTube. https://youtu.be/B1UUjQr5eY8?si=