Tangolight is correct. In order to get the Mission Report to Gamma Orionis you either need to do some uncompleted Missions ahead of it or Skip them to get it.

According to the wiki Gamma Orionis mission opens when you've done the 'Boldly They Rode' mission of the Cardassian arc. Hmmm... It must've opened opened up after 'Cage of Fire', before the latest FE series? Interesting hmm!