  We report the discovery of MeV-GeV gamma-ray emission by the Fermi-LAT
positionally coincident with the TeV pulsar wind nebula (PWN) HESS~J1554-550
within the host supernova remnant (SNR) G327.1-1.1. The gamma-ray emission is
point-like and faint but significant (> 4 sigma) in the 300MeV-2TeV energy
range. We report here the Fermi-LAT analysis of the observed gamma-ray emission
followed by a detailed multiwavelength investigation to understand the nature
of the emission. The central pulsar powering the PWN within G327.1-1.1 has not
been detected in any waveband; however, it is likely embedded within the X-ray
nebula, which is displaced from the center of the radio nebula. The gamma-ray
emission is faint and therefore a pulsation search to determine if the pulsar
may be contributing is not feasible. Prior detailed multiwavelength reports
revealed an SNR system that is old, tau ~ 18,000yrs, where the interaction of
the reverse shock with the PWN is underway or has recently occurred. We find
that the gamma-ray emission agrees remarkably well with a detailed broadband
model constructed in a prior report based on independent hydrodynamical and
semi-analytic simulations of an evolved PWN. We further investigate the
physical implications of the model for the PWN evolutionary stage incorporating
the new Fermi-LAT data and attempt to model the distinct particle components
based on a spatial separation analysis of the displaced PWN counterparts.
