Gravity waves travel at the speed of light. This was predicted by General Relativity and confirmed a few years ago when
2 neutron stars collided 130 million lights years away. After the gravity waves revealed the position the light from the explosion was soon observed as well. As to why light speed is the universal speed limit I can only guess . My guess is it is because deep down everything is both a particle and a wave and the waves travel at the speed of light. You can’t travel faster than the very stuff from which we are made!