The detection of gravitational waves does not necessarily indicate the existence of a massless particle that rectifies gravity in the same manner as a neutrino does for spin. Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects. They were predicted by Albert Einstein's theory of general relativity and were first directly detected in 2015.
According to our current understanding, gravity is described by the theory of general relativity, which does not involve the exchange of particles like other fundamental forces (such as electromagnetism, where photons are the exchange particles). In general relativity, gravity is understood as the curvature of spacetime caused by mass and energy.
While it is true that particles called gravitons have been proposed in some theoretical frameworks attempting to unify general relativity with quantum mechanics (which governs the behavior of particles and forces at a fundamental level), the existence and properties of gravitons are still highly speculative and have not been confirmed experimentally.
Regarding neutrinos, they are indeed particles with very small masses that interact via the weak nuclear force and are known to have spin. However, neutrinos are not directly related to gravity in the same way as they are related to the weak force. The role of neutrinos in particle physics is distinct from the fundamental nature of gravity as described by general relativity.
In summary, while the detection of gravitational waves has provided strong evidence for the predictions of general relativity, it does not directly imply the existence of a massless particle rectifying gravity in the same manner as a neutrino rectifies spin. The study of gravity and the search for a quantum theory of gravity are ongoing areas of research in theoretical physics.