If I recall, there was notational errors Einstein has made in various works, one particular one, if memory serves, for instance, the origin of inertia had at least four corrections when it was converted into English. My point is, that physicists do tend to write errors in their formulas - but keep in mind, the constancy of the speed of light and gravity having a finite speed equal to that of light speed, was known even before Einstein, so there could be a chance Gerber was aware of it and through a sloppy derivation, still came to the right the answer; in other words, it can happen.
A lot of unpublished works by dead ''giants'' of physics, shows numerous errors as well... just wish I could remember half of them now. It seems to me though, this whole issue of Gerber getting the wrong result because of a problem of setting the speed of light equal to that of gravity, appears to be moot since he still arrived at the right result. That isn't an accident, there was a reason Gerber got the result he did and it has been investigated and supported by other physicists - what I find interesting is that Einstein is very quick to call relativity ''his theory,'' when the theory of relativity was largely created before him, even down to length contractions.
It is true, Einstein is not a pure mathematician and in fact required help from his wife to construct the theory of General Relativity. She was however a pure mathematician so this was fortuitous for him without doubt; it stands that this little detail couldn't be hidden and he cannot really call himself the sole author of General Relativity either, even though we tend to think that these days... damn, even the bending of light round objects had been predicted before Einstein.
All in all, its hard not to be convinced Einstein did some plagiarizing at some point and likely from more than one source. His conclusion that Gerber getting the right result as being insignificant because of what analysts have said concerning how he got his result, seems like nit-picking. The fact still remains, Gerber got the right result and it seems to have annoyed Einstein, for some reason, that this fact came to light.
There is an interesting thread debating this subject at this link (from 2016):
How was the Newtonian mercury perihelion precession calculated?
I took some excerpts of answers provided by Andrew Worsley, a graduate Alumnus of University of London, who's being working in
quantum and gravitational physics and has a number of internationally published papers in these fields.
His current project is 'Black hole physics and dark matter modelling.
1) Einstein first used Gerber's equation to calculate it, famously in November 1915, in a presentation to the Prussian academy of Science,
and called it a "higher degree of approximation".
That is not to say that full GTR is the same as Gerber, it is completely different and totally original and also wrong, whereas Gerber's
equation it seems turns out to be the correct equation.
Ironic but a true story, look up Gerber on wiki then look at Einstein's original papers.
My note here: This true fact was noted by Schwartzchild, when debating with Einstein about his exact solution, by December 1915.
It also generated an ironic response from Hilbert, after conceding Einstein's priority. He remarked his surprise due to the speed at
which Einstein performed the due calculations. The truth was that such set of calculations were done in 1914, when he only had his
Entwurf 2.0. So, he used Gerber's formula which he knew, apparently, since long time and possibly inspired him BY 1907/1908 to
extend his 1905 theory to gravity. There are written proofs, as earlier as such, that the idea of retarded potentials inspired him to
extend his theory to the Mercury perihelion. First records about light bending appeared not before 1910/1911.
2) In actual fact if you take into account time dilation, then Gerber is spot on - but does not break down a the black hole event horizon
And the advance in the perihelion is known at best to 2 dp, at a velocity of orbit of 47,000 m/s, SR wiil have only an effect at the 8 dp level.
So WIki doesn't know SR, and is more than a little biased.
My note here: In the heat of the discussion (66 posts), the critics over Gerber goes more to his "error" predicting light deflection than on errors
in Mercury's advance per century. Also, there is a discussion about Einstein bashing the "long time ago dead" Gerber as a nervous reaction.
The discussion continues, reconstructing the image of Gerber as one of the founding fathers of MOND theories (Modified Newton's Dynamics).
As I wrote before, Gerber's work is celebrated even at NASA as an excellent advance over Newton's action at a distance.
There is more, and I don't enter into a discussion about GTR vs. MOND theories to provide valuable answers.
As I posted in my OP, there is no known positive results applying GTR to Halley's comet or other registered excentric orbiting objects, with
periods larger than Halleys's comet. And MOND theories are champions at this particular topic, or the Mercury's perihelion, Flybys or anomalous