Remove IntegrationPatternType.gateway

The `GatewayProxyFactoryBean` really produces an Inbound Gateway
for downstream flow and its proxy becomes a start of the flow

* Determine a return type of the gateway method to indicate
that `void` one leads to the `IntegrationPatternType.inbound_channel_adapter`
instead of regular `IntegrationPatternType.inbound_gateway`

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/IntegrationPatternType.java

@@ -54,8 +54,6 @@ public enum IntegrationPatternType {
 
 	inbound_gateway(IntegrationPatternCategory.messaging_endpoint),
 
-	gateway(IntegrationPatternCategory.messaging_endpoint),
-
 	splitter(IntegrationPatternCategory.message_routing),
 
 	transformer(IntegrationPatternCategory.message_transformation),
@@ -121,7 +119,6 @@ public enum IntegrationPatternCategory {
 				inbound_channel_adapter,
 				outbound_gateway,
 				inbound_gateway,
-				gateway,
 				bridge),
 
 		message_routing(

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

@@ -939,13 +939,17 @@ private static final class MethodInvocationGateway extends MessagingGatewaySuppo
 
 		private boolean isMonoReturn;
 
+		private boolean isVoidReturn;
+
 		MethodInvocationGateway(GatewayMethodInboundMessageMapper messageMapper) {
 			setRequestMapper(messageMapper);
 		}
 
 		@Override
 		public IntegrationPatternType getIntegrationPatternType() {
-			return IntegrationPatternType.gateway;
+			return this.isVoidReturn
+					? IntegrationPatternType.inbound_channel_adapter
+					: IntegrationPatternType.inbound_gateway;
 		}
 
 		@Nullable
@@ -973,13 +977,21 @@ void setupReturnType(Class<?> serviceInterface, Method method) {
 				this.isMonoReturn = Mono.class.isAssignableFrom(this.returnType);
 				this.expectMessage = hasReturnParameterizedWithMessage(resolvableType);
 			}
+			this.isVoidReturn = isVoidReturnType(resolvableType);
 		}
 
 		private boolean hasReturnParameterizedWithMessage(ResolvableType resolvableType) {
 			return (Future.class.isAssignableFrom(this.returnType) || Mono.class.isAssignableFrom(this.returnType))
 					&& Message.class.isAssignableFrom(resolvableType.getGeneric(0).resolve(Object.class));
 		}
 
+		private boolean isVoidReturnType(ResolvableType resolvableType) {
+			Class<?> returnTypeToCheck = this.returnType;
+			if (Future.class.isAssignableFrom(this.returnType) || Mono.class.isAssignableFrom(this.returnType)) {
+				returnTypeToCheck = resolvableType.getGeneric(0).resolve(Object.class);
+			}
+			return Void.class.isAssignableFrom(returnTypeToCheck);
+		}
 	}
 
 	private final class Invoker implements Supplier<Object> {