Gravity vs. Light

You are not correct in your assumptions. Your hypothesis is known as the "tired light" hypothesis; it was tested and it failed the tests. It doesn't happen. Gravity does indeed affect light (crudely put, light has energy, energy is mass by E = mc², therefore gravity affects light) but light is not a physical thing that can be stretched or compressed like a spring. Changing wavelength is changing energy, not physical size. Gravity does change the wavelength of light under some circumstances, but not in the manner that you propose.For more discussion of tired light see Errors in Tired Light Cosmology.