Which is the best way to import data from Quicken(TM)?
Start by making an account tree in Gnucash. There is a druid to help you, but keep at this stage to very similar names as you have used for categories and accounts in Quicken. Next export your Quicken data to a QIF format. Save this file. From the Gnucash menu select File > Import > Import QIF. Open your QIF file and follow the prompts. Match each category very carefully to your Account tree. Clicking on a highlighted line will allow you to choose from your new account tree. Match as much as you can. Then complete the import process. Look at the results and be prepared to alter the account structure and start again if you have too many “Unspecified” entries.