Concatenate and minify css with Gulp still necessary? I'm using Gulp as part of my theming workflow and was wondering if it was necessary to add the concatenate and minify tasks into Gulp if Drupal already does that when I check the option under Performance.
No, it's not necessary, unless you decide there's an appreciable performance benefit by using the gulp method as well (possible but unlikely).

You'll just be minifying twice otherwise, wasted CPU cycles.