CustomTypeAdapter.java

// Make a custom Gson instance, with a custom TypeAdapter for each wrapper object.
// In this instance we only have RealmList<RealmInt> as a a wrapper for RealmList<Integer>
Type token = new TypeToken<RealmList<RealmInt>>(){}.getType();
Gson gson = new GsonBuilder()
        .setExclusionStrategies(new ExclusionStrategy() {
            @Override
            public boolean shouldSkipField(FieldAttributes f) {
                return f.getDeclaringClass().equals(RealmObject.class);
            }

            @Override
            public boolean shouldSkipClass(Class<?> clazz) {
                return false;
            }
        })
        .registerTypeAdapter(token, new TypeAdapter<RealmList<RealmInt>>() {

            @Override
            public void write(JsonWriter out, RealmList<RealmInt> value) throws IOException {
                // Ignore
            }

            @Override
            public RealmList<RealmInt> read(JsonReader in) throws IOException {
                RealmList<RealmInt> list = new RealmList<RealmInt>();
                in.beginArray();
                while (in.hasNext()) {
                    list.add(new RealmInt(in.nextInt()));
                }
                in.endArray();
                return list;
            }
        })
        .create();

// Convert JSON to objects as normal
List<MainObject> objects = gson.fromJson(json, new TypeToken<List<MainObject>>(){}.getType());

// Copy objects to Realm
realm.beginTransaction();
realm.copyToRealm(objects);
realm.commitTransaction();

example.json

[
    { "name"  : "Foo",
      "ints" : [1, 2, 3]
    },
    { "name"  : "Bar",
      "ints" : []
    }
]  

MainObject.java

public class MainObject extends RealmObject {

    private String name;
    private RealmList<RealmInt> ints;

    // Getters and setters
}

RealmInt.java

public class RealmInt extends RealmObject {
    private int val;

    public RealmInt() {
    }

    public RealmInt(int val) {
        this.val = val;
    }

    // Getters and setters
}