In all images except for the stable channel we have:
ogra@pi3:~$ snap interfaces|grep gpio
pi3:bcm-gpio-0 -
pi3:bcm-gpio-1 -
pi3:bcm-gpio-10 -
pi3:bcm-gpio-11 -
pi3:bcm-gpio-12 -
pi3:bcm-gpio-13 -
pi3:bcm-gpio-14 -
pi3:bcm-gpio-15 -
pi3:bcm-gpio-16 -
pi3:bcm-gpio-17 -
pi3:bcm-gpio-18 -
pi3:bcm-gpio-19 -
pi3:bcm-gpio-2 -
pi3:bcm-gpio-20 -
pi3:bcm-gpio-21 -
pi3:bcm-gpio-22 -
pi3:bcm-gpio-23 -
pi3:bcm-gpio-24 -
pi3:bcm-gpio-25 -
pi3:bcm-gpio-26 -
pi3:bcm-gpio-27 -
pi3:bcm-gpio-3 -
pi3:bcm-gpio-4 -
pi3:bcm-gpio-5 -
pi3:bcm-gpio-6 -
pi3:bcm-gpio-7 -
pi3:bcm-gpio-8 -
pi3:bcm-gpio-9 -
This will land in stable once gadget updates are properly implemented. For now use a beta, edge or candidate image to access RasbperryPi GPIO interfaces. The naming of the interfaces follows the mapping of https://pinout.xyz/#