Yes, Ostia is definitely a goth band because most of their albums belong to the Ethereal Wave genre, which is a core subgenre of goth music. Albums like From The Aether, Between Two Cities, and Strange Birds And The Hundred-Handed are examples of their goth sound.
Some of the other goth artists that are similar to them are Aenima and Dark Orange.
Ostia is located in Australia, active in 1992 — present. You can find more information about Ostia on their GothDB band page.