The media business has initially paid $1m for GAO but will pay up to an additional $7m based on performance. GAO is expected to generate revenue of approximately $4m in the 12 months to March 2010.
GAO will sit under United's existing game developer group portfolio, TechWeb, as it looks to boost its business with online game publishers and with consumer online game websites.
Founded in 2006, the banner advertising agency uses a self-service, auction-based system and places in-game and banner advertising on leading gaming websites using cost-per-click and cost-per-acquisition business models.
With access to 50 million unique users GAO delivers over two billion ad impressions per month through a worldwide network of casual, browser-based and mainstream gaming websites, as well as game community networking sites.
Tony Uphoff, chief executive officer of UBM TechWeb said: "GAO provides us with both revenue growth potential and a new business model."