GH-3120: Identify polling MG as outbound-CA

Fixes https://github.com/spring-projects/spring-integration/issues/3120

The messaging gateway method without arguments works as a
polling channel adapter.
In this case a Messaging Gateway component is present in the end of flow.
Therefore we treat it as an `outbound_channel_adapter`

spring-integration-core/src/main/java/org/springframework/integration/gateway/GatewayProxyFactoryBean.java

@@ -761,6 +761,10 @@ public class GatewayProxyFactoryBean extends AbstractEndpoint
 		MethodInvocationGateway gateway = new MethodInvocationGateway(messageMapper);
 		gateway.setupReturnType(this.serviceInterface, method);
 
+		if (method.getParameterTypes().length == 0 && !findPayloadExpression(method)) {
+			gateway.setPollable();
+		}
+
 		JavaUtils.INSTANCE
 				.acceptIfNotNull(payloadExpression, messageMapper::setPayloadExpression)
 				.acceptIfNotNull(getTaskScheduler(), gateway::setTaskScheduler);
@@ -941,15 +945,18 @@ public class GatewayProxyFactoryBean extends AbstractEndpoint
 
 		private boolean isVoidReturn;
 
+		private boolean pollable;
+
 		MethodInvocationGateway(GatewayMethodInboundMessageMapper messageMapper) {
 			setRequestMapper(messageMapper);
 		}
 
 		@Override
 		public IntegrationPatternType getIntegrationPatternType() {
-			return this.isVoidReturn
-					? IntegrationPatternType.inbound_channel_adapter
-					: IntegrationPatternType.inbound_gateway;
+			return this.pollable ? IntegrationPatternType.outbound_channel_adapter
+					: this.isVoidReturn
+							? IntegrationPatternType.inbound_channel_adapter
+							: IntegrationPatternType.inbound_gateway;
 		}
 
 		@Nullable
@@ -992,6 +999,11 @@ public class GatewayProxyFactoryBean extends AbstractEndpoint
 			}
 			return Void.class.isAssignableFrom(returnTypeToCheck);
 		}
+
+		private void setPollable() {
+			this.pollable = true;
+		}
+
 	}
 
 	private final class Invoker implements Supplier<Object> {

spring-integration-core/src/test/java/org/springframework/integration/gateway/GatewayProxyFactoryBeanTests.java

@@ -31,7 +31,7 @@ import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
-import org.junit.Test;
+import org.junit.jupiter.api.Test;
 import org.mockito.Mockito;
 
 import org.springframework.beans.factory.BeanFactory;
@@ -43,6 +43,7 @@ import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.core.convert.support.GenericConversionService;
 import org.springframework.expression.common.LiteralExpression;
+import org.springframework.integration.IntegrationPatternType;
 import org.springframework.integration.annotation.Gateway;
 import org.springframework.integration.annotation.GatewayHeader;
 import org.springframework.integration.channel.DirectChannel;
@@ -56,6 +57,7 @@ import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.PollableChannel;
 import org.springframework.messaging.handler.annotation.Header;
 import org.springframework.messaging.support.GenericMessage;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -156,6 +158,11 @@ public class GatewayProxyFactoryBeanTests {
 		Message<String> message = service.getMessage();
 		assertThat(message).isNotNull();
 		assertThat(message.getPayload()).isEqualTo("foo");
+
+		MessagingGatewaySupport messagingGatewaySupport =
+				proxyFactory.getGateways().get(ClassUtils.getMethod(TestService.class, "getMessage"));
+		assertThat(messagingGatewaySupport.getIntegrationPatternType())
+				.isEqualTo(IntegrationPatternType.outbound_channel_adapter);
 	}
 
 	@Test
@@ -259,6 +266,12 @@ public class GatewayProxyFactoryBeanTests {
 		TestService service = (TestService) proxyFactory.getObject();
 		String result = service.requestReplyWithPayloadAnnotation();
 		assertThat(result).isEqualTo("requestReplyWithPayloadAnnotation0bar");
+
+		MessagingGatewaySupport messagingGatewaySupport =
+				proxyFactory.getGateways()
+						.get(ClassUtils.getMethod(TestService.class, "requestReplyWithPayloadAnnotation"));
+		assertThat(messagingGatewaySupport.getIntegrationPatternType())
+				.isEqualTo(IntegrationPatternType.inbound_gateway);
 	}
 
 	@Test