I'm investigating what licenses are used in Data.gov portal. How I can get the list of the licenses from the portal?
The licenses for the data accessible through Data.gov vary, sometimes based on the source (Federal government, local government, or university) or on previous agreements that led to the data collection or publishing.
In general, the licenses are governed by Project Open Data and will reflect any issue with reuse and redistribution. Some of the licenses used are standardized, while others may be particular to a specific dataset.
On each dataset's page, there is a "license" field in the metadata that should answer your question about the use or reuse of that dataset. For example, NOAA provides a dataset of earthquakes for the last 4100 years! Look at the dataset page, scroll to the bottom and expand the metadata (click "Show More"). The license field notes, "Produced by the NOAA National Geophysical Data Center. Not subject to copyright protection within the United States."
If there is not license noted, the default is that the data is in the public domain and available to use freely and without restriction ("the state of belonging or being available to the public as a whole, and therefore not subject to copyright").