Gutenberg select font family

Not in the way you want. It’s up to you theme to display various elements in one font or another. However, what you can do is assign a custom CSS class (under “Advanced” block sidebar section) to a particular block. Then add corresponding CSS rules to the Additional CSS customizer section.

Typically this would affect the entire block, but if there is some further demarcation within the block which can be targeted with CSS (em or strong tags for example), the font could be applied only to text with such demarcation.