Why Was A Gratian Coin Minted In Siscia?

They minted coins in the east and west for all emperors, from the start of the Tetrarchy (when the Empire was split) e.g. both Constantius Chlorus and Galerius have coins from London and Antioch.

Part of the reason was to show the emperor in charge of that part of the Empire recognised the authority of the other emperor. This happens a lot in Roman coinage e.g. Carausius using AVGGG on his coins to recognise Diocletian and Maximian; and Magnus Maximus (a Western usurper) putting Theodosius (from Constantinople) on his coins.

AFAIK it wasn't until after the full split, in 395 AD, that western mints mostly stopped making coins for eastern emperors (by then the western mints were essentially limited to just Rome and Ravenna); and eastern mints (which were more active and more numerous overall) continued making significant amounts of coins for western emperors for at least a decade after that.
(Sporadic cross-split issues continued into at least the 420s, and possibly even later, but, not counting the mysterious Cherson AE2s, there were no longer a significant fraction of the output.)

I'm not actually sure which side of the empire contained Siscia after 395. It appears that by the middle of the 5th century Siscia was firmly Ostrogothic, but I couldn't figure out where it was before that; it seems to have been close to the boundary.