I said he would have no way to calculate Power Levels because of the inflation in said ratio, which is something that happens in the Namek Saga, out of nowhere. His statistics couldn't compensate for something he wasn't aware of, and they don't feature this inflation in them.
In his calcs, he would notice Goku went from x to 19x (x being the initial power level in the Raditz Arc, and Gero's ""energy reading equivalent""), going from 5-C to 5-B, then he estimated Goku going from 19x to 1846x (which would result in 768k using PLs)
19.230769230769 and 1846.1538461538, to be specific.
In Power Levels, the ratio inflates after the 90000 mark, resulting in High 5-A+ by the time it reaches 540,000.
In Dr. Gero's calculations, given he doesn't know about the inflation, his calculations would have to be based off the progression Goku made from x to 19x
Goku was 1.24x10^29 J (5-C) at x
Goku was 8.292688e+31 J (Low 5-B) at 19x
So, in Gero's point of view, each passing year, Goku would increase his power by 8,280288E31 Joules, like he did before. Power Levels don't have to take part in this in any shape, way or form.
So, (8,280288E31×5)+1,24x10^29 = 4,141384E32, 99 Zettatons (5-B).
In an actual, unbiased, from a person unaware of the functionality of Power Level's perspective, realistically, they'd expect Goku to be Planet level.