Try this (yet unreleased) image:
//cdimage.ubuntu.com/ubuntu-core/16/stable/20181010.1/ubuntu-core-16-armhf+raspi3.img.xz
this has an updated gadget which should have all gpio interfaces listed in snap interfaces
Try this (yet unreleased) image:
//cdimage.ubuntu.com/ubuntu-core/16/stable/20181010.1/ubuntu-core-16-armhf+raspi3.img.xz
this has an updated gadget which should have all gpio interfaces listed in snap interfaces