"Installing" a package with Guix really just means that it is referenced through a link. /gnu is just a cache for "evaluated" package recipes. You could create a profile that contains your symlinks to the items in the store wherever you want, including the currenty directory.

    guix package -p $(pwd)/.guix-profile -i what ever you want

Programmes in this profile can then be executed like so:

    ./.guix-profile/bin/what

But that's not necessary when you could just use `guix environment` and enter a shell where everything is already hooked up.