Greco-Buddhism, as its name suggests, is Buddhism with Greek influence, not vice versa.

It's an example of the intermingling of two different schools of thought. In the wiki link there's no determination on the primacy of influence, but it does tell that Greek populations were taking on Buddhist ideas (indeed 'converting' to buddhism) before the birth of stoicism. In that light, the notion that these ideas traveled back to Greece doesn't seem that far-fetched.