HOW is the Gallbladder attached to the liver
1. Gallbladder attachement to the liver

In this video (check from 55 sec on), you can see that the gallbladder is covered by a thin sheath that wraps the gallbladder; the sheath extends to the liver and intestine and thus keeps the gallbladder in place. The sheath is a part of the abdominal membrane (peritoneum) that covers most of the abdominal organs. You can see that the surgeon is cutting the sheath.

2. The Cystic Vein: The Significance of a Forgotten Anatomic Landmark (PubMed Central)


  ...there is no cystic vein per se, but the venous drainage is
  through the bed of the liver [between the attachment surfaces of the
  gallbladder and liver]...however..Gray's
  Anatomy of the Human Body...describes cystic veins which join at the
  neck of the gallbladder to form either single or double cystic veins
  which flow along the cystic duct and upward along the hepatic
  ducts.