Gabbro ( ) refers to a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to basalt, being its coarse-grained analogue. (Wikipedia)(See all definitions)
In 2006 they released their findings: After drilling through nearly a mile of sediment and crust, they finally hit the gabbro layer for the first time. (discovermagazine.com)
I suspect that the number being used is a lower-bound for the amount of heat released by the oceanic crust because it represents the typical surface heat loss of the great slabs of relatively cool basalt and gabbro. (wattsupwiththat.com)
The textbook explanation is that the Moho draws the line between the crust and the mantle: a demarcation between familiar igneous surface rocks - such as granites, basalts and gabbros - and those of the interior peridotites. (bbc.com)