Slowly, deep underground
The igneous rock gabbro is coarse grained and non-vesicular. Both characteristics show that it is an intrusive igneous rock. Intrusive igneous rocks are formed below Earth's surface and slowly (because they're underground). When rocks are intrusive, they are are non-vesicular (no air pockets) because the air has had the time to escape because of the slow cooling time period. Intrusive rocks are also coarse because crystals had the time to form in the rock during that longer cooling time.
Ps. extrusive igneous rocks form rapidly at Earth's surface and basically have tge opposite characteristics for the opposite reasons (ex: vesicular cause air had little time to escape)