How does GAP compute the dimension of a matrix algebra over the rational numbers? I am curious about the run time.

For example, the manual https://www.gap-system.org/Manuals/doc/ref/chap62.html does the calculation in 62.2-2.

I tried looking at the source code, reference and libraries, but I could not find it. If possible, I would like a reference to the specific location in source of the calculation and a description of what it is doing.