But the Gemini pattern keeps itself going by continuously reconstructing itself, in *spite* of the way the universe normally works.
I was hoping someone would ask that. Let me start out with a comparison to other cellular automata. Conway's Life is B3/S23 -- "born if 3 neighbors, survives if 2 or 3 neighbors". There are other rules, such as HighLife (B36/S23, very close to Conway's Life) in which a 12-cell pattern can replicate itself -- after 12 generations there are 2 copies, after 36 ticks there are 4 copies, and so on. This pattern regularly evolves from random starting states.
There's even a rule, Fredkin's parity rule (B1357/S1257) where every possible pattern is a replicator -- an extreme example of replication being "just the way the universe works". But these replicators are, in some sense, too simple to be very interesting! They replicate the way crystals grow, and it's hard to harness that kind of low-level behavior. If you wanted a HighLife replicator with 13 cells, or one that would replicate in 13 ticks, instead of 12, you'd be out of luck. By comparison, the Gemini spaceship is extraordinarily adjustable.
will this pattern repair itself if anything happens to it? will it protect itself from outside influences? like a cell wall protects the inside of a cell?
No to all of the above. Conway's Life is not amenable to error-correction of this kind, because small changes have such huge consequences. Kind of like building machinery out of chunks of sub-critical enriched uranium: you can design it so that during normal operation the various pieces never come close enough together to start a chain reaction, but if any little thing goes wrong, you end up with high-energy particles flying all over the place, spreading the reaction to other nearby machinery, which then contributes to the explosion.
so how is it reconstructing itself in spite of the things around it?
Well, I didn't say "in spite of the things around it" -- it was "in spite of the way the universe normally works." The Life universe, for random patterns anyway, normally settles into a scattering of stable or P2-oscillating ash after a few hundred or a few thousand generations. There are any number of "lucky" self-perpetuating stationary and moving patterns that are exceptions to this general rule, but they're all very delicately balanced on the edge of chaos.
how is this anything but a different kind of glider?
The Gemini spaceship contains a large amount of data in its glider channels that is recognizably information about its own structure. Change that data, and the replicator unit will (usually) build something different. Most other gliders and spaceships in Conway's Life don't have anything like this -- all the other hundreds of patterns in Golly's Spaceships folder, or the tens of thousands in Koenig's Life Object database, are "naturally" self-perpetuating, because a future generation of the pattern happens to be identical to the original.
The Gemini spaceship has a significantly higher degree of control over its environment: with the right change to its program, a Gemini replicator unit could construct anything that can be built by colliding gliders, in any empty space in the Life universe. The Gemini contains most or all of the construction tools that a Conway's Life self-replicator will need; it's just a few short steps away from being a true replicator. Mostly it just doesn't have the right program -- yet.
There are a few other large patterns, especially Gabriel Nivasch's Caterpillar, that blur this line to some extent. However, the pi-climber "data" in the Caterpillar is much more difficult to reprogram than the gliders in the Gemini. Several new variants of the Gemini with different speeds and angles of travel have already been built -- with a lot of help from the Python scripts that Andrew Wade made available along with the pattern..