It feels odd the first time: you board at Ċirkewwa, travel to Gozo, and pay or validate later. That is the ticketing system, not a loophole.
One ticket, two directions
Gozo Channel says its standard non-subsidised ticket covers one round trip. Foot-passenger and vehicle-plus-driver tickets are validated at Mġarr, Gozo. You can purchase a standard ticket online or at the relevant Mġarr booth. A ticket does not reserve a place on a specific ferry; boarding remains first come, first served.
Children, residents and other eligible travellers can have different fares or ticket requirements. Even an infant travelling free needs the appropriate free ticket. For current prices and categories, use Gozo Channel’s fare page, not a number saved from an old travel blog.
Fast ferries use a different fare structure. Compare the options on the ferry desk, or read fast ferry versus car ferry.