Tonga opened the scoring with an early try and the lead changed hands three times before Newcastle secured victory.
Josh Matavesi started for Fiji as his side cruised to an 18-0 lead after 33 minutes. However, Gloucester were in no mood to roll over at Kingsholm and narrowed the deficit to 18-10 at the break.
The hosts battled hard during the second-period and eventually secured a 31-29 victory. Warriors centre Ravai Fatiaki came on as a replacement and impressed for the visitors.
Fiji now face Ireland A this Saturday while Tonga's next match is against Scotland a week later.