The "Accounts" and "Categories" Tabs

Each line in the Accounts tab display represents a mapping from a Quicken account to a GnuCash account. Similarly, the Categories tab display shows mappings from Quicken categories to GnuCash accounts. Only QIF accounts referenced by one or more transaction records are displayed. The name of the GnuCash account is displayed in "full name" format, including the names of all parent accounts separated by your default separator character (generally ":").

The first thing to check is the column of Quicken account names. Make sure there are no duplicates with slightly-different names. If a QIF transaction makes a transfer to [My Checking], and you imported a file called my-checking.qif, you might have one account entry for "my checking" and one for "My Checking". If these are the same account, you need to go back to the Files tab and reload my-checking.qif with the correct Quicken account name, My Checking.

Once you have all the Quicken accounts making sense, check the GnuCash account column. The default GnuCash account for a given Quicken account is determined by a fallback procedure which makes the best guess it can given the available information. The guesses that are tried are (in order of preference):

Check both the name of the GnuCash account for each QIF account and the type. If you are unhappy with either, click on the row in the display containing the offensive mapping. You will see the Account Picker dialog which will allow you to change it.