Gulp instead of PX gives units of measurement in PC, where to dig?

Question

css

Subject.
I write in SASS.
Task looks like this:

gulp.task('style', function () {
  gulp.src(path.src.style)
    .pipe(plumber())
    .pipe(sass())
    .pipe(prefixer({browsers: ['last 4 versions', 'ie 8']}))
    .pipe(minifyCSS())
    .pipe(rename({suffix: ".min"}))
    .pipe(gulp.dest(path.build.css))
    .pipe(reload({stream:true}));
});

I don’t know where to dig and who is to blame, gulp or sass.
Googling didn't work either.

Answer 1

Blame minifyCSS.
But why are you not satisfied with peaks?