Light is affected by gravity, but not in its speed. General Relativity (our best guess as to how the Universe works) gives two effects of gravity on light. It can bend light (which includes effects such as gravitational lensing), and it can change the energy of light. But it changes the energy by shifting the frequency of the light (gravitational redshift) not by changing light speed.

Gravity bends light by warping space so that what the light beam sees as “straight” is not straight to an outside observer. The speed of light is still constant.

Gravitational potential and the gravitons that carry it travel at the speed of light. So, even if you could instantaneously change the mass of the Sun, there would be an 8-minute delay before it would be noticed at the Earth. It’s all in the theory of General Relativity, which is our best understanding of the way gravity works.