Where does “湯加” come from?

湯加 (Chinese) comes from English Tonga, from Tongan Tonga, from Samoan toga — south; southern.

Ancestry of “湯加”, step by step

Step	Language	Word	Meaning
1	English	Tonga	—
2	Tongan	Tonga	—
3	Samoan	toga	south; southern

湯加 — Etymology, Origin & Word History | Etymology Explorer