Matrix

This is one of the best all around sites that I have found. It is titled “The MacTutor History of Mathematics Archive.”
http://www-history.mcs.st-and.ac.uk/~history/

This is an award-winning site that links to all sorts of mathematics information.
http://archives.math.utk.edu/

Gauss:
http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Gauss.html

Jordan:… See the whole entry