The crash happened on the A496 near the pumping station at Ty'n y Cefn, between Blaenau Ffestiniog and Maentwrog, at about 17:30 GMT on Tuesday.
A person with life-threatening injuries was flown to North Staffordshire University Hospital, Stoke, and one woman was taken to Ysbyty Gwynedd in Bangor.
The road remains closed on Wednesday morning due to the fuel spillage.
Emergency repairs are also being carried out on the road.
North Wales Police has appealed for witnesses to call 101.
Two people were taken to hospital after a two-car collision in Gwynedd.