Stuart Rojstaczer maintains a web page with information about average GPA at many universities in the U.S. The data on that page was last updated in 2009, and is next scheduled to be updated in February 2016 (according to a note from the author on the page.)
At the end of that page, there are links to the historical GPA at each of the schools for which he has data. It also indicates the source of the data, so you can find out whether the school actually publishes the data, or whether the school simply made the data available to the author.
For example, if I click on his link for University of Wisconsin - Madison, I learn that this data was collected from the registrar's website. While the particular link he cites is no longer alive, searching the registrar's site yields this page that allows me to find out grade distributions from any course and term. Similarly, I found out through his page that UCSB publishes this data through its office of institutional research, which led me to this page of reports that include average GPA by major.
You can follow a similar procedure to find grade distribution information for other schools if available, but it may not be available for the schools you are interested in. Universities are not obligated to publish this information.