gulp-sass @import CSS file

gulp-cssimport is a good choice if you also want css imports to work in non-minified development environments.

Passing some parameters to minifyCSS allows me to achieve this.

.pipe(minifyCSS({
        relativeTo: './bower_components',
        processImport: true
    }))

Sources:
gulp-minify-css depends on clean-css, referenced options explained here.