RS.java

public class RS {
    public static void main(String[] args) throws FileNotFoundException {
        Type t = new TypeToken<List<RateInfo>>(){}.getType();
        Gson gson = new GsonBuilder().registerTypeAdapter(t, new RSDeserializer()).create();

        TopLevel topLevel = gson.fromJson(new FileReader(new File("test.json")), TopLevel.class);
        System.out.println(topLevel);
    }
}

class TopLevel {
    private AVL[] AVL;

    public String toString() {
        return Arrays.toString(AVL);
    }
}
class AVL {
    private Rates RATES;

    public String toString() {
        return RATES.toString();
    }
}

class Rates {
    private List<RateInfo> RS;

    public String toString() {
        return RS.toString();
    }
}

class RateInfo{
    private String BEG;
    private String END;
    private int RATE;
    private String RQ;

    @Override
    public String toString() {
        return "RateInfo{" +
                "BEG='" + BEG + '\'' +
                ", END='" + END + '\'' +
                ", RATE=" + RATE +
                ", RQ='" + RQ + '\'' +
                '}';
    }
}

class RSDeserializer implements JsonDeserializer<List<RateInfo>> {
    @Override
    public List<RateInfo> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
        if (json instanceof JsonArray) {
            return Arrays.asList(context.deserialize(json, RateInfo[].class));
        }
        return Collections.singletonList(context.deserialize(json, RateInfo.class));
    }
}