It is incredibly easy to set up. When you start a new file, you will be asked what set of accounts you would like to create. Examples of the account sets include business accounts, and household accounts. The business account has categories such as Sales and Accounts Payable, while the Household account has categories such as Mortgage and Car Loan. Once you select a set of accounts, you can add or delete categories to meet your particular needs.
GnuCash allows for the import of both .QIF files (used by Quicken) and .OFX (Open Financial Exchange) files. Which means you can download your information from your online banking account and import it directly into GnuCash.
There is detailed documentation. Many open source programs work great if you know what you are doing, but do not have good documentation. The documentation for GnuCash consists of a complete help menu as well as a detailed tutorial. The tutorial even has an overview of basic accounting principals.