Gift sub-type is one of the important fields on the Gift screen and lots of users have stored important information on it. Most of the fields on the gift tab are available for selection on the reports and queries. Unfortunately, Gift Sub-type is the only field that is not available at the any of the reports. It is strongly recommended that this should be part of the selection of the reports, especially on the Pledge and collection related report.
I think this actually is available on some of the reports -- gift detail and summary is one that comes to mind. The problem with canned reports is that they are often not as customizable as you might like. It helps to learn how to use Export to get the info that you need or custom reports for a more advanced option.