In Kotlin, how does GSON.fromJson(...) manage to mutate a read-only field of an existing object?

In Kotlin, how can an instance's read-only val field be mutated?

In the following Kotlin code, gson.fromJson(...) gets a Thing(0) from the InstanceCreator and somehow manages to mutate value from 0 to 1, then return that object. How?

import com.google.gson.GsonBuilder
import com.google.gson.InstanceCreator

class Thing(val value: Int)

fun main() {
    val creator = InstanceCreator { Thing(0) }
    val gson = GsonBuilder().registerTypeAdapter(Thing::class.java, creator).create()
    val thing = gson.fromJson("""{"value":1}""", Thing::class.java)
    println(thing.value) // 1
}

I verified that the object returned by gson.fromJson(...) is the same object provided by the InstanceCreator, so it's not creating a new instance based on the one provided by the InstanceCreator.

I also tried setting value using reflection, but didn't find a way. There was no setter available on the val field.