You have to start gdb with the name of your LilyPond executable.
Ugh, OK, now it's getting confusing.
Since gdb can't handle whitespaces in the path, I copied the whole project directory into a new folder, and rewrote the includes and filenames to eliminate whitespaces. If I try to compile the document in a path with whitespaces, I get the ly:trampoline error; the *exact same file*, in a path without them, fails under different conditions. Also, compiling fails with those errors in frescobaldi, but not when I run lilypond on the file from gdb (so there's no backtrace to post).
So now what?