Interesting and noble concept, though very widely disregarded from even before the time of Krishna.

Where exactly in Gita does Krishna propound this theory? 

If you remember your Mahabharata, Kansha was heavily discriminated, criticized, and accused of all sorts of things by Pandavs - the very dear ones of Krishna.  If Krishna advocated this noble cause, would they not have followed it too?

I believe it was Raja Santanu who started this Sanaatan Dharma that segregated the society in four barnas and so on.  Maybe he intended to make the society more efficient by allocating work as such, but I am sure his intention was more of "divide and rule".

The idea you mentioned above was actually expressed by Buddha, perhaps for the first time in then Bharatvarsha.