GitHub Desktop uses system fonts. When creating design mockups for GitHub Desktop, consider that:
In Figma, use macOS fonts (Primer Web defaults) for mocking up designs, unless the feature is Windows-specific.
Windows' system font is Segoe UI, and type styles for Windows are included in the Figma library to make it easy to switch from SF Pro to Segoe to show the difference between Mac and Windows screens. Segoe UI reads a little smaller and tighter, so it may be worth adjusting the point size to make it feel consistent with macOS.
Styles and sizes follow the same guidelines as Primer Web, with the addition of text-x-small (11pt and used for all small, secondary text).