• Ledger 2.6 can read gnucash 2.4.x files natively
• A python script to convert gnucash 2.6.x sqlite3 files to ledgers file format: https://github.com/MatzeB/pygnucash
• Inspired by the above, piecash added an export option for exporting GnuCash sqlite book to ledger format. This uses piecash's GnuCash data schema.
• A python script to convert gnucash 2.6.x XML file (uncompressed) to ledgers file format: https://gist.github.com/nonducor/ddc97e787810d52d067206a592a35ea7

For GnuCash 3+, the best option might be to export data into CSV and then import into Ledger.