Gabbro is an intrusive, mafic, and igneous rock. How would it turn into sedimentary rock?
The only way for any igneous rock to transform into sedimentary form is through erosion, grain by grain. It won't be done en masse. The most easily erodible parts will dislodge from the surface first.
When the grains have deposited into a sedimentary bed, over time, they will just be grains of minerals. If there are other forms of igneous rock nearby and they erode into the same bed, over the same period, no-one will be able to distinguish a gabbro portion from another host rock portion.