Just look for any 20mm strap/band and it should fit. I’d recommend a band with quick-release pins.
From the manufacturer
AMOLED Display & Editable Widgets
The customized 1.65-inch stylish square screen offers a larger display area than a round watch face of the same width. The key information is displayed centrally, focusing on what matters most to you.
14 Days Battery Life
Amazfit GTS brings about a long-lasting endurance of 14 days while keeping a slim watch body, which enables you to avoid frequent charges.
5 ATM Water Resistance
Water resistant to a depth of 50 meters, Amazfit GTS also supports multiple swimming scenarios.
12 Sports Modes
Amazfit GTS supports 12 mainstream exercise modes. These are outdoor running, walking, outdoor cycling, indoor cycling, treadmill, elliptical trainer, climb, trail run, pool swimming, open water swimming, skiing, exercise.
Slim Metal Watch Body
A smooth transition from the 2.5D curved glass to the metal watch body lowers the thickness to 9.4 mm, which makes it slim and delicate.
PPG Sensor & Realbeats AI-Based Engine
Amazfit GTS can perform constant 24-hour high-precision heart-rate monitoring as well as a heart-rate interval monitoring during workouts, and warning if a much too high value is detected.
Amazfit GTS Fitness Smartwatch with Heart Rate Monitor, 14-Day Battery Life, Music Control, 1.65" Display, Sleep and Swim Tracking, GPS, Water Resistant, Smart Notifications, Desert Gold,Amazfit,W1914OV7N