Make GsonAutoConfiguration align with JacksonAutoConfiguration

See gh-11591

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/gson/GsonAutoConfiguration.java

@@ -43,6 +43,7 @@ import org.springframework.core.Ordered;
 public class GsonAutoConfiguration {
 
 	@Bean
+	@ConditionalOnMissingBean(GsonBuilder.class)
 	public GsonBuilder gsonBuilder(List<GsonBuilderCustomizer> customizers) {
 		GsonBuilder builder = new GsonBuilder();
 		customizers.forEach(c -> c.customize(builder));