Not GP, but there was a huge economy around banner / ads being embedded in Flash games, and those games being shared widely / distributed on many different sites.
There were many "portal" sites that showed (hosted) many games and showed ads alongside or before the games started, and then a second sub-ecosystem of ads embedded in the games themselves by the game developers.
Instead of feeling like their games were "pirated" when somebody copied the game's .swf file, developers had this (viral) mechanism which made those games lucrative wherever they'd appear on the web!
Source: know someone who ran one of those "portal" sites and made an absolute killing.