GraphQL is a query language for your API, and it differs from REST by allowing clients to request exactly the data they need. Unlike REST, which often over-fetches or under-fetches data, GraphQL enables clients to specify their data requirements, resulting in more efficient and flexible data retrieval.

Selecting an appropriate API Gateway is crucial for managing scalability, security, and traffic routing in a scalable application. A well-chosen API Gateway can handle increased traffic, ensure reliability, and provide security features like rate limiting and authentication. It also aids in routing requests to the appropriate microservices.

API development tools like Swagger facilitate adherence to the OpenAPI Specification by automatically generating API documentation from your code. This documentation includes details about endpoints, request/response formats, and authentication, making it easier for developers to understand and interact with the API. This promotes adherence to the specification and helps ensure consistent API design.

API analytics tools help in monitoring data usage to ensure the API is performing optimally. They track how the API's data is consumed, helping identify areas for optimization and resource allocation.

The REST architectural style enforces statelessness in Web APIs by not storing any client state on the server. Instead, each request from a client to the server must contain all the information necessary to understand and process the request. This approach simplifies server-side management and allows for scalability and fault tolerance. Frequent sessions and cookies are not part of REST's stateless design.

Not implementing API versioning can lead to potential issues, as changes to the API may break existing clients that rely on the previous version. API versioning is a crucial practice to ensure backward compatibility and provide a clear way to handle changes and updates.

The most commonly used protocol for Web APIs is HTTP (Hypertext Transfer Protocol). It is the foundation of data communication on the World Wide Web and is used for requesting and transmitting data between clients and servers, making it ideal for web-based APIs.

Ensuring GDPR compliance for an API involves steps such as conducting a security audit to identify vulnerabilities and potential data breaches. GDPR compliance requires a proactive approach to protect user data and ensure that it is handled securely. Encryption, user authentication, and monitoring data access are essential components of GDPR compliance.

Optimizing API performance involves various techniques, such as efficient data formats, minimizing error handling, and utilizing caching to reduce the load on the server. These practices improve response times and resource utilization, making APIs faster and more efficient for users.

Best practices for API testing include Input Validation to ensure that the API behaves correctly, even when faced with invalid or unexpected inputs. Input validation involves checking and filtering incoming data to prevent security vulnerabilities and ensure the API's robustness and security.