Error running gulp sass

Edit 3

Don't forget that with Sass, it doesn't have to be one or another :
you can include / import "as many" files as you wish.

1. Copy the content of your style.css into a style.scss (.scss) file (or maybe a less generic name).
2. At the very beginning, import your ionic.app.css (or you could point to ionic.app.scss) using Sass @import. At the beginning, because you may want to use a global / brutal override on a ionic element/class later on.
3. alter the gulpfile.js to process style.scss and compile it to the proper target file, style.css, in the www/css folder. Just follow the default gulpfile.js, you don't have to be a gulp specialist, just change the origin and destinations paths.