Gulp, while being a great build system, isn't perfect. Its plugin system is immature and not as full-featured as Grunt. I created gulp-grunt as a remedy for this problem, making it possible to import tasks from Grunt to gulp.
However, sometimes you want to use variables in both files. How? Well, use an accessory file to define all your variables, and import it from both.
In this gist is an example(a terrible one at that, but whatever) of importing stuff from an external file.