要在Spring Cloud Gateway中使用WebClient異步調用微服務，可以按照以下步驟進行操作：

1. 添加依賴：在項目的pom.xml文件中添加WebClient和Spring Cloud Gateway的依賴。

<dependencies>
    <!-- Spring Cloud Gateway -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-gateway</artifactId>
    </dependency>

    <!-- WebClient -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
    </dependency>
</dependencies>

2. 創建GatewayFilter：創建一個GatewayFilter來處理請求并使用WebClient來異步調用微服務。

@Component
public class MyGatewayFilter implements GlobalFilter, Ordered {

    private final WebClient webClient;

    public MyGatewayFilter(WebClient.Builder webClientBuilder) {
        this.webClient = webClientBuilder.build();
    }

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // 異步調用微服務
        Mono<ClientResponse> responseMono = webClient.get()
                .uri("http://microservice/service")
                .exchange();

        // 處理響應
        return responseMono.flatMap(response -> {
            // 復制響應狀態碼、頭部信息等
            ServerHttpResponse serverResponse = exchange.getResponse();
            serverResponse.setStatusCode(response.statusCode());
            serverResponse.getHeaders().putAll(response.headers().asHttpHeaders());

            // 轉發響應體
            return response.bodyToMono(String.class)
                    .flatMap(body -> {
                        serverResponse.getHeaders().setContentLength(body.length());
                        return serverResponse.writeWith(Mono.just(serverResponse.bufferFactory().wrap(body.getBytes())));
                    });
        });
    }

    @Override
    public int getOrder() {
        return -1;
    }
}

3. 配置GatewayFilter：在應用的配置文件中配置GatewayFilter。

spring:
  cloud:
    gateway:
      routes:
        - id: my_route
          uri: http://localhost:8080/
          filters:
            - MyGatewayFilter

這樣，當使用Spring Cloud Gateway進行路由時，會自動調用MyGatewayFilter來處理請求并使用WebClient異步調用微服務。