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.

I had a portal like that as a teen (GameGum.com) before they blew up (once people started selling the “one click all in one arcade scripts” it was all over, even though ours was far nicer and had a nice community).

Made good money for a few years and bought my first car off it!

Ads through a network like CPMStar were the majority, averaged about $0.50 per thousand views. At the time there was also a flash game marketplace at fgl.com, that let game portals like Addicting Games or Newgrounds bid to "sponsor" your game which involved adding their branding and links to their site. On my most popular game I made $100k over a few years.

There are still lots of such portals with Flash and HTML5 games - so is that economy still thriving well?

I did two games for fun when I was younger. I only made 200$ in total, but they were made in less than 4 hours each (I did them both in a math class, thus why the time restriction). To monetize them, I used MochiMedia, which sadly died with Flash, and they were the one that distributed them. I got about half a millions views on both of them, mostly from China (which had an ecpm of 1 cent too :')).