I re-read the Foundation series a few months ago but I was just thinking about it now in a case of very late Fridge Logic. If Daneel already established Gaia a long time ago, why did he feel the need to establish psychohistory and the Foundation? Wouldn't Galaxia already be a way to ensure the success of humanity in the universe?
Daneel helped to create psychohistory, because he wasn't sure that Gaia would work.
To quote from Die Rückkehr zur Erde (German Translation):
Unterdessen - vor fünf Jahrhunderten, als es schien mir würde es nie gelingen, all die Schwierigkeiten zu überwinden die der Gründung Gaias im Wege standen - wandte ich mich der zweitbesten Lösung zu und half mit, die Wissenschaft der Psychohistorik zu auszulösen.
Short english translation:
Five centuries ago, when it seems that I never could solve the problems of Gaias birth, I also implemented the second best solution and helped to create the science of psychohistory.
So he has done it, because he didn't know if Gaia would work.
Quote from Die Rückkehr zur Erde, Heyne Verlag ISBN 3-453-52018-1 Page 544 (German translation of "Foundation and Earth")
Daneel's mission, due to the three laws of robotics embedded in him (plus the zeroth law), was to ensure that humanity survives. His vision was a humanity that could better handle the inherent chaos of the universe. To reach this goal he formulated two approaches, a humanity guided by Psychohistory; and as an alternative, humanity as the super organism Galaxia (of which Gaia is a prototype).
Daneel couldn't predict which approach would work better so he initiated both of them, so that if one path leads to a dead end the other path would still have hope.
As it was apparent in Foundation and Earth, Daneel is continually tweaking his plans as he learns more and humanity itself develops. It is hinted that both approaches would eventually merge (and perhaps the genetic modification approach used by Solaria) into one grand solution of the human condition.
In the ending of Prelude to Foundation, Daneel talks to Hari Seldon. Daneel says that besides psychohistory, he is also working on another plan, far from this one, because he can't be sure either one will work. In Prelude to Foundation, this explains part of why Daneel is so busy, because besides having to be the first minister of the emperor Cleon, he also works with Gaia.
Daneel does not tell what his other plan is, but we learn about Daneel's connection with Gaia in Founation and Earth, and some more details in David Brin's Foundation's Triumph.