Android application that consumes WebService

I'm trying to create my first app to make use of a WebService, in that case I'm just trying to return some of the information via string and later I'll put each information in its proper component, I'm trying to use the coinmarketcap API to bring but when I try to convert JSON the error: java.util.concurrent.ExecutionException: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was BEGIN_ARRAY at line 1 column 2 path $ appears, follow mine 3 app classes so far:

public class MainActivity extends AppCompatActivity {
//ATRIBUTOS
private Button btOK;
private TextView tvResposta;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btOK = (Button) findViewById(R.id.bt_consultar);
    tvResposta = (TextView) findViewById(R.id.tv_resposta);

    btOK.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            //FAZENDO A CONSULTA NO WEBSERVICES
            try{
                Coins coins = new HTTPService().execute().get();
                tvResposta.setText(coins.toString()); //aqui chama a função to string

            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }
    });
}
} 

Template class:

public class Coins {

private String id;
private String name;
private String symbol;
private String rank;
private String price_usd;
private String price_btc;
//private String volume_usd_24h;
private String market_cap_usd;
private String available_supply;
private String total_supply;
private String percent_change_1h;
private String percent_change_24h;
private String percent_change_7d;
private String last_updated;

public String converter(){
    return "id: "+ getId()
            +"\n name: "+ getName()
            +"\n rank: "+ getRank()
            +"\n price_usd: "+ getPrice_usd()
            +"\n price_btc: "+getPrice_btc();
}

Class requesting and connecting:

public class HTTPService extends AsyncTask<Void, Void, Coins> {


@Override
protected Coins doInBackground(Void... voids) {
    StringBuilder resposta = new StringBuilder();

    try{
        //URL QUE SERÁ CONSUMIDA
        URL url = new URL("https://api.coinmarketcap.com/v1/ticker/bitcoin/");

        //---- ABRINDO A CONEXÃO ---
        HttpURLConnection conexao = (HttpURLConnection) url.openConnection();
        conexao.setRequestMethod("GET");
        conexao.setRequestProperty("Content-type","application/json");
        conexao.setRequestProperty("Accept", "application/json");
        conexao.setDoOutput(true);
        conexao.setConnectTimeout(5000);
        conexao.connect();

        //----- LENDO AS INFORMAÇÕES -------
        Scanner scanner = new Scanner(url.openStream());

        while (scanner.hasNext()){
            resposta.append(scanner.next());
        }

    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    //---- convertendo dados do JSON
    return new Gson().fromJson(resposta.toString(), Coins.class);
}

@Override
protected void onPostExecute(Coins coins) {
    super.onPostExecute(coins);
}

@Override
protected void onProgressUpdate(Void... values) {
    super.onProgressUpdate(values);
}
}

The error is displayed when you get to this line here:

return new Gson().fromJson(resposta.toString(), Coins.class);

Note: I omitted the getters and setters of the template class, but they are all there.