I doubt if Retrofit should support kotlin Result at this moment, because returning kotlin Result from functions is not the default behavior.

Anyway, this call adapter will work for your use case.
note:
That call adapter will work for suspend fun foo() : Result<Foo> , but won't work for fun foo(): Result<Foo> in some reasons.

Initially I thought that might be a good addition, but I guess you can always use runCatching if you want the type wrapped in Result

val user: Result<User> = runCatching {
    api.getUser()
 }

val user: Result\u003CUser> = runCatching {\n api.getUser()\n }

You can try my solution - https://github.com/agamula90/RetrofitKx, hosted on mavenCentral, based on retrofit + moshi + ksp if you are are looking for better kotlin api