Gratian is also an option, but in this case, we can narrow it to Valens or Valentian I. Valens being the strongest candidate.Many emperors used the same or very similar reverse, including Valens, Valentian I, Theodosius I, Arcadius, Constans, Constantine II and Julianus; no doubt even more. Specific ID of 'your' emperor may be a challenge. The image on the reverse is called: GLORIAROMANORVM and can be researced in that manner.
Buona fortuna,
Don.