Fonts
Gio’s text shaper uses the type []text.FontFace to represent the collection of available fonts.
There is one font bundled in package gioui.org/font/gofont, you can use gofont.Collection() to get a []text.FontFace containing all of the variants of the Go fonts.
For loading other fonts there is gioui.org/font/opentype. After parsing the font(s) using opentype.Parse, you can append them to a []text.FontFace.
Shapes
For converting strings to clip shapes there is the gioui.org/text package.
It contains text.Cache that implements cached string to shape conversion, with appropriate fallbacks. Simply provide your fonts ([]text.FontFace) to text.NewCache.
In most cases you can use widget.Label which handles wrapping and layout constraints. Or when you are using material design material.LabelStyle.