It was known since the 1850s that the perihelion precession of Mercury is a few percent larger than predicted by Newtonian mechanics. This even led to speculations that there might be another undiscovered planet ("Vulcan") in a close orbit around the sun. In 1916 Einstein calculated that this effect could be fully explained by General Relativity. Another famous prediction is that of the deflection of light that passes close to a star. This was confirmed by Eddington during the 1919 solar eclipse.

Light deflection happens in newtonian physics as well, assuming you use Newton's definition of light as a particle/corpuscule: https://physics.stackexchange.com/questions/29837/does-newto...

It's just 2x the GR value. That's one reason why it took so long to verify as you needed to have pretty clean data to rule out the classical explanation.

I don't think that anyone took the corpuscular theory of light seriously after its wave nature had been discovered in the early 19th century. In classical electrodynamics, light is completely unaffected by gravity.