def setGS(gs):
    """Set fixation bias, if `gs` is given."""
    if gs > 0:
        selgamma = ['global shape2;\n', 'category rateCatSel =(' + str(gs) +
            """, EQUAL, MEAN, GammaDist(_x_,shape2,shape2), CGammaDist(_x_,shape2,shape2),0,1e25);
"""
            ]
    else:
        selgamma = ['rateCatSel := 1.0;\n']
    return selgamma
