I think it's a bit generous to consider GenDesign the developer of TLG and that GenDesign was formed by former members of Team Ico.
Out of the 700 people (519 developers, 181 thanks) listed in the TLG game credits, only 14 of them are from GenDesign. Out of these 14 GenDesign staff listed there, only 6 of them worked in previous Japan Studio games (in most cases only a handful games), the other ones joined GenDesign being TLG their first game or joined from different non-Sony studios.
Around half a dozen outsourcing companies worked in this game, not counting GenDesign, audio, PR, marketing, localization, etc.
The majority of Japan Studio devs who worked on TLG stayed at Japan Studio after the game and some worked in Knack 2, Demon's Souls and mostly the Astro Bot games. Now are at Team Asobi or the XDEV Asian team that resulted from the restructuring of Japan Studio that separated their internal 1st party development from the 2nd party publishing of Asian games divisions of the studio into two separate studios.
Some Japan Studio devs who worked on TLG left Sony during or after the development of TLG. Many of them haven't released a game since then, other ones joined other companies where did work on games like Pokemon or Smash Bros.