How to convert map and nested map to string and vise versa in Kotlin?

I have no idea how to convert HashMap and nested Hashmap to string and vise versa:

I use Room to save data in a local database. The "users " hashmap is a parameter of the data class. To save this parameter "users " in the Room database, we must convert it to a primitive such as string, boolen, Integer, etc. Room does not support object references, so type converters are required.

val users = HashMap<String, HashMap<String, Any>>()

    val user1 = HashMap<String, Any>()
    user1["filter"] = 0
    user1["group"] = "groupId"
    user1["labelsVisibility"] = true
    user1["sort"] = 1
    user1["view"] = 3
    users["user1"] = user1

    val user2 = HashMap<String, Any>()
    user2["filter"] = 0
    user2["group"] = "groupId"
    user2["labelsVisibility"] = true
    user2["sort"] = 1
    user2["view"] = 3
    users["user2"] = user2

As I said earlier in the comments, it could be easily done with Gson library:

// Configure Gson
val gson = GsonBuilder()
    // Using this strategy Gson first tries converting number to Long. 
    // If it fails (number is float), then it tries converting to Double
    .setObjectToNumberStrategy(ToNumberPolicy.LONG_OR_DOUBLE) 
    .create()

// Convert to JSON string
val jsonString = gson.toJson(users)
println("JSON => $jsonString")

// Store at Room as a string...
// Fetch later from Room as a string...

val type = object : TypeToken<HashMap<String, HashMap<String, Any>>>() {}.type
val fromJsonMap: HashMap<String, HashMap<String, Any>> = gson.fromJson(jsonString, type)

println("ORIGINAL MAP =>\n$users")
println("RESTORED MAP =>\n$fromJsonMap")

Output:

JSON => {
  "user1": {
    "filter": 0,
    "view": 3,
    "labelsVisibility": true,
    "sort": 1,
    "group": "groupId"
  },
  "user2": {
    "filter": 0,
    "view": 3,
    "labelsVisibility": true,
    "sort": 1,
    "group": "groupId"
  }
}

ORIGINAL MAP =>
{user1={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}, user2={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}}
RESTORED MAP =>
{user1={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}, user2={filter=0, view=3, labelsVisibility=true, sort=1, group=groupId}}

As you can see both original Map users and restored Map fromJsonMap are identical.