Go proportional fonts are sans-serif, like several popular fonts for screen displays. There is some evidence that some sans-serif faces at small sizes and low resolutions on screens are slightly more legible than their seriffed counterparts, while at large sizes, there is not a significant difference in legibility between sans and seriffed faces, at least in the pair tested.
Go sans-serif fonts are "humanist" rather than "grotesque" in style. This is an historical distinction, not an aesthetic judgment. Widely used sans-serif fonts like Helvetica and Arial are called grotesque because an early 19th century sans-serif typeface was named "Grotesque," and the name became generic.
Go proportional italics have the same width metrics as the roman fonts. Go italics are oblique versions of the romans, with one noticeable exception: the italic lowercase 'a' is redesigned as a cursive single-story form to harmonize with the bowl shapes of the b d g p q set, in which the upright forms also adapt well to slanting, The addition of cursive 'a' makes the italics appear more lively than a simply slanted roman.
The Go proportional fonts come in three weights: Normal, Medium, and Bold. The Normal weight is strong enough that it maintains clarity on backlit screens, which often tend to erode letter features and thickness. The Medium weight has stem thickness 1.25 times the Normal, for greater sturdiness on bright screens or for users who prefer a sturdy font.