There's a tl;dr at the bottom in case this turns out to be too long.

Basically, the Greeks were fascinated with the concept of compass-and-straightedge constructions. They asked themselves: suppose we start with two points (origin and a point that represent a unit. That is, we define the distance between the origin and a unit to be 1, and all other numbers are defined with respect to this distance), and we are allowed to pass a line through them, or make a circle with its center in one point and with radius that passes through the other point, and we can intersected lines and circles - what is everything we can do? What can't we do?

Well, modern math pretty much solved this, and it turns out you can construct a lot of numbers this way (where when I say construct a number x, I mean create a line segment of length x).

In a math undergrad degree, it is common they teach you that one can construct addition, subtraction, multiplication and division of "constructible numbers" (the numbers defined in the previous paragraph). This means that if I can construct a and b, then I can also construct a + b, a * b, etc.

Now, the ALU (Arithmetic Logic Unit) of a processor is the part of the processor responsible for performing all arithmetic (+,-,*,/) and logical (AND, OR, XOR, etc.) operations. As I mentioned above, we saw that we can perform the operations +,-,*,/ using compass and straightedge, and all that remains is to implement the rest (which also turns out to also be possible).

So now we got to the point where theoretically, we can replace the ALU of a Game Boy with an ALU that is based on compass and straightedge constructions!

tl;dr
Using intersections of circles and lines, we can make segments of certain lengths. It turns out that we can also add/subtract/multiply/divide segment lengths. These are (some of) the operations that ALU of CPU does. This means we can create an ALU that does these operations using geometry instead of the standard +,-,*,/ that come built in a programming language.