I have two gonothrombium species (Both of them includes larvae obtained from the adult female by experimental rearing). The tarsal claw formula 2-2-3 or 3-3-3 for gonothrombium larvae. I don't know for sure.
There is no any key but only three species of Gonothrombium are described in the world as follows: Gonothrombium bequaerti (Fuller et Wharton, 1951) [L] ; Gonothrombium bimaculatum Feider, 1950 [P, L] and Gonothrombium oudemansianum (Feider, 1948) [P, L]. You can check your species with them.