A series of very graphic adult Gravitation doujinshi by the original mangaka:

Panda パンダ (Yuki uke, Seguchi x Yuki x Tatsuha) (2006)
Kumagorou くまごろう (Shuichi x Yuki) (2006)
Capybara カピバラ (Eiri x Shuichi / Hiro x Suguru ) (2007)
Sakana 魚 (2013)
Hyoko ひよこ (2013)
Ushi 牛 (201...