No, because Justified Image Grid adapts to the font used by your theme. It's best to be consistent about fonts. Let the theme manage the content font and subsequently, how the gallery captions look like. There are no font-family CSS rules in JIG.
In case your theme doesn't come with an option to use Google Fonts, here is how you do it:
- Install Easy Google Fonts plugin (it's free).
- Go to Settings > Google Fonts screen.
- Add JIG to the Control name field and hit Enter.
- Add .jig-caption CSS selector then click Save Font Control.
- Go to the Customizer as it suggests. It's at Appearance > Customize by the way.
- Navigate to a page or post that has a JIG gallery (with captions).
- Go to Typography > Theme Typography and click Edit Font under JIG.
- Select a nice font under Font Family, such as Roboto or Source Sans Pro.
- Click Save & Publish once done.
Note: Set all other aspects such as color, background color, opacity, size, align, shadow, CSS rules such as font-weight in JIG, as it's unlikely to override those styles with Easy Google Fonts.