The building that the Alterians/Drath (Biosphere Modulator?) get that adds about 3 tiles to the planet works perfectly fine, so I was taken back when the Gaia Vortex didn't work
The Gaia Vortex gives a bonus to the Planet Quality ability (+10% is about 1 tile'ish), which hasn't ever worked for planets you already colonized.
However, it also seems to be intended to give 10 tiles to the planet it is built on... but it uses "PlanetQualityBonus" (something that is only used by the Terraforming improvements...) instead of "PlanetQualityChange" like Weather Control and such do.
Speaking of that PlanetQualityChange thing... I really wish that it wouldn't be removed if an agent was placed on a building that gives it (but the other features of said building would). There is potential in that ability to make things like faux-global bonuses of things you couldn't otherwise make global bonuses for. Or global bonuses that have to be established first on each world first, much like Soil Enhancement and the other terraforming improvements work.
For instance, put a PlanetQualityChange bonus of 1 on a planetary improvement that gives +10% Planetary Defense, make a tech to give it... and, there you go! You effectively have made a tech that gives a +10% bonus to Planetary Defense to all planets you control without taking up any planetary tiles (at least after the improvement is built, anyway). Sure, it has a side effect of increasing morale slightly, but *shrugs*
The problem with doing this, though, is that, you can't let these buildings be disabled either... otherwise, that planet-quality change will also disappear. I'm only guessing this is the case, but since all the PlanetQualityChange buildings also can't have agents placed on them... it's a reasonable assumption.