Add WebFlux SSE support with GSON

Closes gh-35884

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/codec/json/GsonEncoder.java

@@ -37,6 +37,7 @@
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.HttpMessageEncoder;
+import org.springframework.http.codec.ServerSentEvent;
 import org.springframework.util.Assert;
 import org.springframework.util.FastByteArrayOutputStream;
 import org.springframework.util.MimeType;
@@ -106,7 +107,8 @@ public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType
 		if (!super.canEncode(elementType, mimeType)) {
 			return false;
 		}
-		return !String.class.isAssignableFrom(elementType.toClass());
+		Class<?> elementClass = elementType.toClass();
+		return !(String.class.isAssignableFrom(elementClass) || ServerSentEvent.class.isAssignableFrom(elementClass));
 	}
 
 	@Override

spring-web/src/main/java/org/springframework/http/codec/support/ServerDefaultCodecsImpl.java

@@ -58,6 +58,7 @@ protected void extendObjectWriters(List<HttpMessageWriter<?>> objectWriters) {
 	private @Nullable Encoder<?> getSseEncoder() {
 		return this.sseEncoder != null ? this.sseEncoder :
 				(JACKSON_PRESENT || JACKSON_2_PRESENT) ? getJacksonJsonEncoder() :
+				GSON_PRESENT ? getGsonEncoder() :
 				KOTLIN_SERIALIZATION_JSON_PRESENT ? getKotlinSerializationJsonEncoder() :
 				null;
 	}

spring-web/src/test/java/org/springframework/http/codec/json/GsonEncoderTests.java

@@ -23,6 +23,7 @@
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.testfixture.codec.AbstractEncoderTests;
+import org.springframework.http.codec.ServerSentEvent;
 import org.springframework.web.testfixture.xml.Pojo;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -42,7 +43,8 @@ protected void canEncode() throws Exception {
 		assertThat(this.encoder.canEncode(pojoType, APPLICATION_JSON)).isTrue();
 		assertThat(this.encoder.canEncode(pojoType, APPLICATION_NDJSON)).isTrue();
 		assertThat(this.encoder.canEncode(pojoType, null)).isTrue();
-
+		assertThat(this.encoder.canEncode(ResolvableType.forClass(ServerSentEvent.class), null)).isFalse();
+		assertThat(this.encoder.canEncode(ResolvableType.forClass(String.class), null)).isFalse();
 	}
 
 	@Test