Big caveat that I’ve never seriously tried using it, but my perspective is that GNUCash and GNU Ledger have different target audiences. The way I think of it is (Cash is to LibreOffice) as (Ledger is to some combination of Vim and SQLite).
Cash aims to replace non-free accounting software, Ledger is both a file format and a general purpose tool for parsing and transforming that file format, that is designed for accounting but can be used to drive a wide variety of applications.
Cash aims to replace non-free accounting software, Ledger is both a file format and a general purpose tool for parsing and transforming that file format, that is designed for accounting but can be used to drive a wide variety of applications.