What is the speed of gravitational waves ?
Gravitational waves travel at the speed of light.

There are many possible sources of gravitational waves. The two confirmed detections so far (14 Sep 2016) are merging black hole binary systems, but sources of (detectable) gravitational waves may include merging neutron stars, short period binaries containing neutron stars or white dwarfs, gamma ray bursters or supernovae.

For a brief introduction see Gravitational Wave Sources at the ALIGO site.