In 2017, 41.5 million smartwatches will be sold. They are on pace to account for the highest unit sales of all wearable device form factors from 2019 to 2021, aside from Bluetooth headsets. By 2021, sales of smartwatches are estimated to total nearly 81 million units, representing 16 percent of total wearable device sales.

"Smartwatches are on pace to achieve the greatest revenue potential among all wearables through 2021, reaching $17.4 billion," said Angela McIntyre, research director at Gartner. Revenue from smartwatches is bolstered by relatively stable average selling prices (ASPs) of Apple Watch.

"The overall ASP of the smartwatch category will drop from $223.25 in 2017 to $214.99 in 2021 as higher volumes lead to slight reductions in manufacturing and component costs, but strong brands such as Apple and Fossil will keep pricing consistent with price bands of traditional watches," she added.