Yes, gabbro and basalt are similar chemically and in composition. The rocks are made mostly of plagioclase and pyroxene minerals. The rocks may also contain augite and olivine. Both are igneous rocks, which means that they are formed by the cooling and solidification of either magma or lava.
Gabbro is intrusive, which means it is found inside Earth's crust. However, basalt is usually extrusive. When referring to rocks, the term "extrusive" implies that the rock is found above Earth's surface.
Another difference between basalt and gabbro has to do with their grains, or the size of the crystals within the rocks. Basalt is fine-grained whereas gabbro is coarsely-grained. The difference in the crystal sizes of the rocks is due to the rate at which the lava or magma cools. Being intrusive, gabbro is closer to Earth's hot inner core. Therefore, it takes longer for gabbro to cool.