A gun is a repeating factory
A glider gun is a periodic pattern that fires gliders. The Gosper glider gun is the famous first example: 36 live cells in a 36 x 9 bounding box, period 30, discovered by Bill Gosper in 1970.
Every 30 generations the core reaction returns while a new glider escapes. The gun does not stabilize into ash. It keeps increasing the total live-cell history by exporting motion.
Why this mattered
Before a gun, a finite Life seed could plausibly be expected to settle into still lifes, oscillators, and escaping leftovers. The Gosper gun gave a concrete counterexample: a finite starting pattern with unbounded growth.
That changed the emotional center of Life. The game was no longer only about watching what a seed becomes. It was about building mechanisms that keep producing signals.
What to watch in the simulator
The gun is easier to understand if you ignore the full board at first. Watch the two stable blocks, then the central collision, then the departing glider stream.
After one full period, the core should look like it did at the start, apart from the glider that has moved away. That is the signature of a working gun.
Working takeaway
Load the gun, run to generation 30, pause, and compare the core. The repeated core plus escaped glider is the whole story.