Gun and Run is an origin trait in Destiny 2 that can appear on 24 weapons. There is also an enhanced version with a stronger effect.
What Gun and Run does
Defeating targets builds stacks of Gun and Run for a short duration. Sprinting for a short time consumes a stack to partially reload this weapon's magazine from reserves; on Bows, this briefly improves draw speed.
Enhanced version
Defeating targets grants stacks of Gun and Run for a longer duration. Sprinting for a short time consumes a stack to partially reload this weapon's magazine from reserves; on Bows, this improves draw speed for longer.