"The Great Gatsby" BluRay disc should play ok on all BluRay players, especially newer ones.
If you are having difficulty getting it to play on the DMP-BDT120, then there may be a firmware glitch.
Try doing a factory reset on the DMP-BDT120. If the disc still won't play, then try updating the firmware.
Full instructions on how to download and install the firmware update are on the Panasonic website: http://panasonic.jp/support/global/cs/bd/download/bdt220/bdt220_euro.html