What it desperately needs is a quickstart of some sort, here's this package which has these dependencies and he's how you can use buildstream to build it.

I gleaned some info from Tristan's blog post which was pretty good,
Once you have built a stack in BuildStream it should be trivial enough to deploy it as rpms, debian packages, a tarball/ostree SDK sysroot, as a flatpak, or as a bootable filesystem image which can be flashed to hardware or booted in a VM."

Can buildstream build packages for multiple platforms? The docs don't give an answer to that, or would you have to use something like https://github.com/jordansissel/fpm

An example could be gcc, is it possible to build gcc, resolving all dependencies and create a package for different platforms with different sysroots? So an rpm, deb etc.

I was reading the comments to Tristans blog last night https://blogs.gnome.org/tvb/2017/02/06/introducing-buildstream-2/

He mentions "The output of this build will be a bootable image which can be launched in qemu/kvm." This would be very useful for testing builds where a clean environment is required