gdb has been a rat's nest of difficult code due to a million difficult corner cases (even before you take into account all the crazy oddball systems it supports which just increases the complexity combinatorially). By comparison a program like gcc is conceptually "simpler" since the majority of its code base is devoted to the problem at hand (code analysis and generation).

gdb's codebase been continuously painful for as long as I had to deal with it (about the decade before 2000). And 99.99% of my "dealing" with it has been indirect. Yeoman's work was put in by John Gilmore and Stu Grossman in the early-mid 90s both who somehow managed to avoid burning out. My hat is off to Pedro Alves for not only being willing to work on gdb but to tackle gnarly problems.

Just to be clear, pace Shakespeare, I post to praise gdb, not bury it. A debugger is one of the most complex pieces of systems code possible.