The main goal of this paper is to solve the Nordstrom-Einstein paradox in the General Theory of Relativity (GTR) and open a new perspective on understanding the generation of gravitational fields. At present it is not exactly known how the gravitational field is generated. This field is responsible for attraction at the distance of bodies in Newton’s sense and for space-time distortion in Einstein’s sense. In 1913, Nordstrom blamed Einstein’s GTR that it cannot explain why the electromagnetic waves confined in a mass-less box with reflective walls have a gravitational mass while the invariant of the stress-energy tensor of the electromagnetic field is zero. This is the Nordstrom-Einstein paradox in GTR. The fact that his paradox was not solved for 111 years created a niche in GTR which blocked progress in the field of gravity.The present paper solves the Nordstrom-Einstein paradox for the hypothetical case of a sphere with a mass-less reflective surface filled with electromagnetic waves. It demonstrates that the gravitational field of such a sphere is generated only during the reflection of electromagnetic waves by the sphere’s reflective surface and not in its volume. The methodology used is specific to GTR, establishing a new form of the stress-energy tensor for the case of the mentioned hypothetical sphere. Solving this paradox has significant implications for understanding the gravitational field essence. It can serve as a basis for further theoretical and experimental research, with important implications for the development of Quantum Gravity, String/Superstring theory, and Unified Field theories.