On Windows, your Highlights Gallery will be stored in:
\Users\[username]\Videos\NVIDIA\GeForce NOW/[game]
On macOS your Highlights will be stored in:
/Users/[username]/Movies/NVIDIA/GeForce NOW/[game]
Where are the Highlights videos and screenshots stored?
Updated:
Where are the Highlights videos and screenshots stored?
\Users\[username]\Videos\NVIDIA\GeForce NOW/[game]
/Users/[username]/Movies/NVIDIA/GeForce NOW/[game]