Looks like you need a PC copy of Portal:
https://github.com/lambertjamesd/portal64/releases
(There's some older releases below as a ready-to-go .z64 rom image, though, which should be ready to run on an Everdrive)
https://github.com/lambertjamesd/portal64/releases
(There's some older releases below as a ready-to-go .z64 rom image, though, which should be ready to run on an Everdrive)