1. Key Difference 1: Design Philosophy - Google Test (also known as gTest) is designed to support C++ testing, while NUnit is designed specifically for .NET languages like C#. This means that gTest provides more features and flexibility for C++ developers, including advanced mocking and parameterized tests. NUnit, on the other hand, focuses on providing a simple and intuitive testing framework for .NET applications.
2. Key Difference 2: Assertions and Matchers - Google Test uses assertions to check if a condition is met, while NUnit uses matchers to verify test expectations. Assertions in gTest can include custom failure messages and have more advanced features such as death tests. NUnit matchers provide a more expressive assertion syntax that allows for more detailed test failure messages.
3. Key Difference 3: Testing Frameworks and Dependencies - Google Test is a standalone testing framework that can be used independently of other tools or frameworks. NUnit, on the other hand, is often used in conjunction with other testing frameworks like Selenium or Unity for automated testing in .NET applications. NUnit also provides built-in support for data-driven tests using external data sources.
4. Key Difference 4: Test Fixture Setup - In Google Test, the test fixture setup code is run before each individual test, ensuring a clean and isolated state for each test case. NUnit, on the other hand, allows for more flexibility in test fixture setup by providing setup methods that can be executed once for multiple tests or for the entire test fixture.
5. Key Difference 5: Test Discovery and Execution - Google Test uses a test discovery mechanism that relies on macros to identify test cases and automatically generate test runners. NUnit uses reflection to discover and execute test cases, providing a more dynamic and flexible approach to test discovery.
6. Key Difference 6: Continuous Integration and Report Generation - Google Test provides built-in support for generating XML-based test reports that can be easily integrated into continuous integration systems like Jenkins. NUnit also supports XML-based test reports, but it also offers more advanced features like category-based test selection and parallel test execution to optimize test runs.

In Summary, Google Test is a feature-rich testing framework designed for C++ development, while NUnit is a simpler and more focused framework for testing .NET applications, providing a more expressive assertion syntax and integration with other tools and frameworks.