Где я могу применить onTap в следующем коде

GestureDetector(
              onTap: (){},
              child: Container(
                height: MediaQuery.of(context).size.height / 6,
                child: ListView.builder(
                  primary: false,
                  scrollDirection: Axis.horizontal,
                  shrinkWrap: true,
                  itemCount: categories == null ? 0 : categories.length,
                  itemBuilder: (BuildContext context, int index) {
                    Map cat = categories[index];
                    return Padding(
                      padding: EdgeInsets.only(right: 10.0),
                      child: ClipRRect(
                        borderRadius: BorderRadius.circular(8.0),
                        child: Stack(
                          children: <Widget>[
                            Image.asset(
                              cat["img"],
                              height: MediaQuery.of(context).size.height / 6,
                              width: MediaQuery.of(context).size.height / 6,
                              fit: BoxFit.cover,
                            ),
                            Container(
                              decoration: BoxDecoration(
                                gradient: LinearGradient(
                                  begin: Alignment.topCenter,
                                  end: Alignment.bottomCenter,
                                  // Add one stop for each color. Stops should increase from 0 to 1
                                  stops: [0.2, 0.7],
                                  colors: [
                                    cat['color1'],
                                    cat['color2'],
                                  ],
                                  // stops: [0.0, 0.1],
                                ),
                              ),
                              height: MediaQuery.of(context).size.height / 6,
                              width: MediaQuery.of(context).size.height / 6,
                            ),
                            Center(
                              child: Container(
                                height: MediaQuery.of(context).size.height / 6,
                                width: MediaQuery.of(context).size.height / 6,
                                padding: EdgeInsets.all(1),
                                constraints: BoxConstraints(
                                  minWidth: 20,
                                  minHeight: 20,
                                ),
                                child: Center(
                                  child: Text(
                                    cat["name"],
                                    style: TextStyle(
                                      color: Colors.white,
                                      fontSize: 20,
                                      fontWeight: FontWeight.bold,
                                    ),
                                    textAlign: TextAlign.center,
                                  ),
                                ),
                              ),
                            ),
                          ],
                        ),
                      ),
                    );
                  },
                ),
              ),
            )