I don't think that you can calculate the temperature dependence of the vibrational frequencies in Gaussian. What you can do is calculate anharmonic contributions to the vibrations and then calculate thermochemical contributions to the the free energy and look at the temperature dependence of, say, the vibrational partition function. Some information regarding thermochemistry in Gaussian can be found here: http://www.cup.uni-muenchen.de/ch/compchem/G98thermo.pdf