"L2 is stable like a pin balanced on its point is stable; ie. not very much."
L2 is a pretty broad pin. The annual delta-V consumption to maintain an L2 halo orbit is 30 - 100m/s, which compares well with stationkeeping in geostationary orbit (50-55m/s), low Earth orbit (25m/s for a 400km orbit like the ISS), or low lunar orbit (0 to 400m/s). There are a lot of gravitational influences out there to make any orbit imperfectly stable, and sometimes other effects (e.g., low Earth orbit's atmospheric drag.)
"I'm also curious why L2?"
As a sensitive observatory, Gaia is going to L2 to get away from Earth's heat, light, radio noise, and view obstruction.
Heat: Gaia is depending on passively cooling down quite a bit (hence that sunshield parasol), and being in low Earth orbit would bring a lot of infrared heat from multiple angles (since Earth is a big disk to objects in LEO, not a point source). While an astronaut wouldn't feel toasty from Earth's radiating heat, it's enough to make passive storage of mild cryogenic liquids (e.g., liquid oxygen) difficult, or to passive cool an astronomical satellite's instruments. Moving to L2 reduces the cooling problem to the sun, which is a point source easily hidden behind a modest sunshield. (Even heating from zodiacal light can be a problem for REALLY sensitive instruments or passive liquid hydrogen storage, though I don't think Gaia cares.)
Light: glare off Earth is another bother for optical satellites like Gaia. Moving to L2 reduces Earth to (almost) a point source.
Obstruction and orbital problems: As noted above, Earth is a big object to the perspective of a satellite in LEO, blocking out more than a third of the sky. Short orbital periods means satellites are constantly having their views altered, and tidal effects can stabilize (or tumble) satellites in undesired ways. Gaia might find it annoying to be trying to aim at Ceti Alpha 6 when Earth is like, "LOL! Nope, time for gravient gradient stabilization!"
Noise and other disturbances: There's a lot of radio chatter and time spent dodging orbital debris in low Earth orbit.
Higher Earth orbits present radiation issues from the Van Allen Belts or, in the case of geostationary orbits, a lot of radio chatter and crowded traffic lanes. Geostationary is cluttered to about its limit before communication satellites cause radio cross-talk and interference problems.
So: L2. There's no traffic to speak of, no radio interference, no brutish Earth blocking views, no debris to dodge, much simpler tidal / gravity issues, and vast, empty vistas for Gaia to work with.