First of all, this is purely a thought experiment. The width of Gimli isn't even a power of two (384 bits), and secondary storage bus speeds aren't even worth using a high performance permutation like Gimli. So from a practical perspective, this is totally pointless.
But it's nonetheless an interesting one. And from it, I have three concrete questions that can be considered:
Can we build a tweakable bijective keyed permutation from just a pseudo-random forward permutation?
Is there a practical disk encryption system using only forward permutation?
How do we interpret the "new conventional wisdom" that "a permutation is a better unified primitive than a block cipher"?
Gimli paper can be here.