1. Key Difference 1: Protocol Support: Envoy is a versatile proxy that offers support for a wide range of protocols, including HTTP/1.1, HTTP/2, gRPC, and TCP. In contrast, Google Cloud Load Balancing primarily supports HTTP/1.1 and HTTP/2 protocols, although it can also handle TCP and SSL traffic to some extent. This difference in protocol support makes Envoy a more suitable choice for scenarios that require flexible protocol handling or involve modern communication patterns like microservices and service mesh architectures.

2. Key Difference 2: Traffic Management Capabilities: Envoy provides advanced traffic management capabilities, such as load balancing, circuit-breaking, health checks, and retries out of the box. It also supports request shadowing and fault injection for testing and debugging purposes. On the other hand, Google Cloud Load Balancing mainly focuses on basic load balancing functionality with features like global and regional load balancing, SSL termination, and auto-scaling. While it provides scalability and availability, Envoy's comprehensive traffic management features make it a preferred choice for complex traffic routing and control scenarios.

3. Key Difference 3: Extensibility and Customization: Envoy offers a highly extensible architecture with a wide range of filters and plugins that can be used to modify and customize its behavior. This allows users to add custom logic and integrate with various external systems seamlessly. In contrast, Google Cloud Load Balancing provides limited customization options, primarily relying on predefined load balancing algorithms and configurations. The extensibility of Envoy makes it a more suitable choice for organizations that require fine-grained control over their traffic management and routing logic.

4. Key Difference 4: Service Discovery and Registry: Envoy supports multiple service discovery mechanisms, including DNS, static configuration, and dynamic discovery via service registries like Google Kubernetes Engine (GKE) and HashiCorp Consul. It can dynamically discover and load balance requests to backend services based on their availability and health status. Google Cloud Load Balancing, on the other hand, relies on the integration with Google Cloud Platform services like Compute Engine (GCE) and Kubernetes Engine (GKE) to discover and manage backend services automatically. This difference gives Envoy an edge in scenarios where multi-cloud or hybrid deployments are involved, allowing seamless integration with various infrastructure setups.

5. Key Difference 5: Observability and Metrics: Envoy provides extensive observability features, including support for distributed tracing with OpenTracing, metrics collection using the StatsD format, and logging with integration to various logging systems. It offers detailed insights into request flow, latency, error rates, and other performance indicators. While Google Cloud Load Balancing also provides basic monitoring and logging capabilities, it does not offer the same level of observability and fine-grained metrics provided by Envoy. Envoy's observability features make it a preferred choice for organizations that require deep insights into their application's performance and troubleshooting.

6. Key Difference 6: Deployment and Ecosystem: Envoy has gained significant adoption in the industry and has a thriving community that contributes to its development and maintenance. It is part of the Cloud Native Computing Foundation (CNCF) and benefits from a rich ecosystem of tooling and integrations. On the other hand, Google Cloud Load Balancing is a Google Cloud Platform-specific offering and, while widely used within the platform, may not enjoy the same level of community support and ecosystem integrations as Envoy. The broader deployment and ecosystem support of Envoy make it a more versatile and future-proof choice for organizations looking for long-term stability and interoperability.

In Summary, Envoy offers broader protocol support, advanced traffic management capabilities, higher extensibility and customization, versatile service discovery mechanisms, extensive observability, and benefits from a larger deployment and ecosystem compared to Google Cloud Load Balancing.