Catalysis of the reaction: 8-oxo-7,8-dihydroguanosine triphosphate = 8-oxo-7,8-dihydroguanosine phosphate + diphosphate. 8-oxo-7,8-dihydroguanosine triphosphate (8-oxo-GTP) is the oxidised form of the free guanine nucleotide and can act as a potent mutagenic substrate for transcription.