Webflow preloads some Google Fonts into it's Designer's font selection, for example Montesserat. However, there are only a few font weights available in the preloaded section and it is impossible to add additional weights through the "Settings > Fonts" interface.
For example, I want to use Monteserrat Light. A font weight which isn't available in the Designer's preloaded font list. So I head on over to the Site Settings and Fonts tab to choose the additional Monteserrat weights. However, Monteserrat isn't on the list.
I can of course head to fonts.google.com and insert the code manually but the font should be available from Webflows usual Google Font loader interface.