  We discuss the high enegry afterglow emission (including high energy photons,
neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR
1806-20. If the initial outflow is relativistic with a bulk Lorentz factor
\Gamma_0\sim {\rm tens}, the high-energy tail of the synchrotron emission from
electrons in the forward shock region gives rise to a prominent sub-GeV
emission, if the electron spectrum is hard enough and if the intial Lorentz
factor is high enough. This signal could serve as a diagnosis of the initial
Lorentz factor of the giant flare outflow. This component is potentially
detectable by GLAST if a similar giant flare occurs in the GLAST era. With the
available 10 MeV data, we constrain that \Gamma_0 < 50 if the electron
distribution is a single power law. For a broken power law distribution of
electrons, a higher \Gamma_0 is allowed. At energies higher than 1 GeV, the
flux is lower because of a high energy cut off of the synchrotron emission
component. The synchrotron self-Compton emission component and the inverse
Compton scattering component off the photons in the giant flare oscillation
tail are also considered, but they are found not significant given a moderate
\Gamma_0 (e.g. \leq 10). The forward shock also accelerates cosmic rays to the
maximum energy 10^{17}eV, and generate neutrinos with a typical energy
10^{14}eV through photomeson interaction with the X-ray tail photons. However,
they are too weak to be detectable.
