I was looking at the page where I once downloaded the Japanese GP2 iso but it was hacked.
From what I found out, only dumped roms in mame format are available.
However, that format is not compatible with the dolphin triforce emulator, but it seems that there is a tool to transform mame files into iso format so that the emulator can read them, also remember to change the header so that the emulator can read it correctly.
Probably these tools help to create iso formats of triforce games through mame formats:
https://github.com/ChainSwordCS/triforce-combine-decrypt
https://github.com/FIX94/triforce-header-patcher
I assure you that I have no idea how they work, but I hope this helps you.