A developer working with Gatsby should possess a combination of frontend development skills, modern web technologies, and a good understanding of static site generation and the JAMstack architecture. Here are the key skills a Gatsby developer should have:

1. React: Gatsby is built on top of React, so a strong understanding of React fundamentals, including component-based development, state management, and JSX syntax, is essential.
2. HTML and CSS: Proficiency in HTML and CSS is crucial for creating and styling website layouts and components.
3. JavaScript (ES6+): A solid grasp of modern JavaScript features and syntax is needed to work effectively with Gatsby’s frontend codebase.
4. GraphQL: Gatsby uses GraphQL for data querying. Familiarity with GraphQL concepts, querying data, and understanding schemas is important.
5. Static Site Generation (SSG): Understanding the principles and benefits of static site generation, as well as when to use it versus server-side rendering (SSR), is key.
6. JAMstack Architecture: Knowledge of the JAMstack architecture, which involves using JavaScript, APIs, and Markup, is essential for building efficient and scalable Gatsby applications.
7. Version Control (Git): Proficiency in using Git for version control is crucial for collaborating on codebases and managing changes.
8. Responsive Web Design: Ability to create websites that look and function well on various screen sizes and devices.
9. Performance Optimization: Knowledge of techniques to optimize website performance, such as lazy loading, image optimization, code splitting, and minimizing HTTP requests.
10. Webpack: Basic understanding of Webpack, the build tool used by Gatsby under the hood, is helpful for customizing build processes.
11. Command Line: Proficiency in using the command line to run development and build scripts, manage dependencies, and interact with Gatsby’s CLI.
12. Package Managers: Familiarity with package managers like npm or Yarn for installing and managing project dependencies.
13. API Integration: Ability to integrate data from various sources through APIs, whether they are third-party services or custom APIs.
14. Debugging and Troubleshooting: Strong problem-solving skills to debug issues and troubleshoot errors in both code and build processes.
15. Basic Design Principles: A grasp of basic design principles to ensure visually appealing and user-friendly interfaces.
16. CMS Integration: Understanding how to integrate Gatsby with headless CMS platforms for efficient content management.
17. SEO Best Practices: Knowledge of search engine optimization (SEO) principles and how to implement them effectively in Gatsby projects.
18. Continuous Integration/Continuous Deployment (CI/CD): Familiarity with CI/CD pipelines and automating deployment workflows.

Assessing a Gatsby developer’s proficiency involves various methods:

1. Technical Evaluation: Conduct interviews and coding challenges on Gatsby-related topics.
2. Portfolio Review: Evaluate their past Gatsby projects and coding quality.
3. Problem-Solving: Present hypothetical Gatsby problems to gauge their solutions.
4. Code Review: Analyze their feedback on a Gatsby project’s codebase.
5. Live Sessions: Assess skills in real-time coding or pair programming.
6. Technical Assessments: Use online platforms for Gatsby-specific tests.
7. Project Discussion: Explore their previous Gatsby projects and experiences.
8. Communication: Evaluate how they explain concepts and document work.
9. Behavioral Interviews: Ask about teamwork, adaptation, and time management.
10. Open-Ended Questions: Pose queries to understand Gatsby concepts and workflows.

Hiring a capable Gatsby developer involves several key steps:

1. Clear Requirements: Define the job description and skills needed.
2. Technical Proficiency: Evaluate expertise in Gatsby-related technologies.
3. Portfolio Review: Examine previous Gatsby projects and quality of work.
4. Problem-Solving: Assess problem-solving abilities through challenges.
5. Communication Skills: Gauge effective communication for teamwork.
6. Team Fit: Ensure compatibility with the team’s culture.
7. Adaptability: Consider willingness to learn new tools.
8. References: Contact provided references for insights.
9. Coding Challenges: Incorporate technical assessments.
10. Cultural Alignment: Check values and work ethics match.
11. Remote Work Experience: Assess remote collaboration ability.
12. Continuous Learning: Evaluate openness to staying updated.
13. Domain Knowledge: Prior experience in specific domains can be valuable.
14. Compensation: Discuss salary transparently.
15. Trial Period: Offer a trial or contract period if possible.

Some key advantages include its fast speeds due to preloading assets during builds; its modern approach towards SEO friendly content through static site generation; plus its wide range of plugins allowing seamless integration across multiple services without having write custom code each time.

Two must haves being ReactJS & GraphQL which form part of Gatsby’s underlying architecture providing access points for manipulating data stored either locally or remotely via APIs respectively . Other popular libraries worth mentioning include Styled Components, Emotion JS, Apollo Client amongst others.