I am curious: what scholars have argued that the "Gallic Empire" coins are unofficial in total? Because to me, the definition of "official coins" is rather easy: coins are official in the territory of the issuing authority, unless somehow restricted by that authority.
The term "usurper", on the other hand, is tricky.
Gallienus was the son and co-ruler of Valerian, who was an usurper.
Tetricus was the successor of
Victorinus, who, in turn, was the successor of
Marius and
Postumus, also an usurper. So who is usurper, who is not? That's the wrong question, originating from a modern point of view. In the 3rd Century AD, being a legitimate emperor is mainly based upon your own
success and
auctoritas and much less so on your birth alone, legalistic claims or the senate (just take
Aurelian as example!). Valerian
had success and gained
auctoritas, which made him the "legitimate"
Augustus for seven years. But then he was defeated and captured by Shapur, and
his son
Gallienus was questioned by many "usurpers" not in the least because he was, in the eyes of many, apparently lacking
success and
auctoritas.
Lars