Kepler discovered the curvature of space,
with his three orbital constraints;
Gauss measured it with a theodolite of his own construction
in Allsace-Lorraine -- it is not perfectly convex.

GR is "just" a manifestation of gravity,
what ever it really is. what it is definitely not, though,
is any perfect "pascalian" vacuum (with no pascals .-)

GR's prediction for the 'anomaly': 42.98 +/- 0.04
is well inside the error bar.

Not quite. *All these effects on Mercury’s orbit including GR one if
indeed exists are not linearly additive.