Just because you think something is true does not make it so.
Gurren Lagann's characters all come from very well defined molds and we don't even consider their pasts.
We're given the characters in their present forms and they interact and take on Genome and later the Anti-Spirals.
Code Geass spends far more time on characterization in that it fleshes out the characters and their relationships to one another in the past as well as their flaws and strengths in far more detail than Gurren Lagann.
The only character in Gurren Lagann who got a lot of development was Simon, a few others changed somewhat over the course of the series like Viral, Genome, and Rossiu, but none of them was as fleshed out as say Suzaku or Lelouch.
Code Geass gets the obvious advantage in characterization.
Gurren Lagann gets the obvious advantage in maintaining a derived and consistent plot.