Paul Gerber published in 1898 and 1902 a theory for the precession of Mercury's orbit, including this formula:
Ψ = 24 π3 a2 / (τ2 c2 (1 - ε2))This is identical to what Einstein published in 1915. The difference is that Einstein based it on relativity, and Gerber just assumed that the speed of gravity was the same as the speed of light.
It is considered a consequence of relativity that gravity propagates at the speed of light, and since Gerber did not know relativity, he must have made some other hidden assumptions.
Here is Einstein's 1920 repudiation:
Mr. Gehrcke wants to make us believe that the perihelion shift of Mercury can be explained without the theory of relativity. So there are two possibilities. Either you invent special interplanetary masses. [...] Or you rely on a work by Gerber, who already gave the right formula for the perihelion shift of Mercury before me. The experts are not only in agreement that Gerber’s derivation is wrong through and through, but the formula cannot be obtained as a consequence of the main assumption made by Gerber. Mr. Gerber’s work is therefore completely useless, an unsuccessful and erroneous theoretical attempt. I maintain that the theory of general relativity has provided the first real explanation of the perihelion motion of Mercury. I did not mention the work by Gerber initially, because I did not know about it when I wrote my work on the perihelion motion of Mercury; even if I had been aware of it, I would not have had any reason to mention it.Einstein is famous for not citing prior work, and here we see him defending the practice. He says that he would not cite Gerber's correct formula, because his derivation was not a real explanation.
So maybe this is why he didn't cite Lorentz or Poincare or Hilbert or others whose work he plagiarized. He was claiming priority for the first real explanation, and did not want to dilute that with an acknowledgement of prior work.