Here’s a link to a .reg file with the defaults if you need it.
Usual disclaimer: Use at your own risk. Backup your registry before making any changes.
Computer\HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics
Computer\HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics
IconSpacing and IconVerticalSpacing both are string values
Default: -1125
Min: -480
Max: -2730
Tip: You can copy and paste into the address bar in registry editor.