Go to Theme Settings >> Select Typography section.
You can set font-family, font-weight, font-style & font-size for: Body, Heading & Sub Title.
Result:
+ Body Font Settings:
+ Heading Font Settings:
+ Sub Title Font Settings:
NOTE: Gecko only support font of Google. If you want to use Customize font, you can use Fontify – Customize Fonts for your Shopify store please!