For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the mean of the pKas of this molecule.
The pH of an electrophoretic gel is determined by the buffer used for that gel.  If the pH of the buffer is above the pI of the protein being run, the protein will migrate to the positive pole (negative charge is attracted to a positive pole).  If the pH of the buffer is below the pI of the protein being run, the protein will migrate to the negative pole of the gel (positive charge is attracted to the negative pole).  If the protein is run with a buffer pH that is equal to the pI, it will not migrate at all. This is also true for individual amino acids.