A Giant Molecular Cloud (GMC) is a Molecular Cloud (i.e., including compounds such as Molecular Hydrogen (aka HH or H2), with a mass on the order of 103 to 107 solar masses. They are assumed to be a common place for Star Formation, pretty much necessary.
Bound GMCs (bound by Gravity) are observed, but most appear to be Unbound. An area of research is how gravitationally bound volumes that become stars form within an unbound GMC.
Molecular Cloud Turbulence
Orion Molecular Cloud Complex