1. 1. Language Support: One key difference between Buck and Gradle is the language support they offer. Buck is primarily designed for building projects written in Java, Android, and C/C++, whereas Gradle supports a wider range of languages including Java, Kotlin, Groovy, Scala, and more.
2. 2. Build Speed: Another major difference lies in the build speed of these build systems. Buck is known for its fast and efficient incremental builds, as it only rebuilds the necessary dependencies. On the other hand, Gradle may suffer from slower build times, especially for large projects, due to its more generic and flexible model.
3. 3. Configuration Style: Buck and Gradle also differ in their configuration styles. Buck uses a declarative approach, where users define the build rules explicitly in a Buck configuration file. In contrast, Gradle uses a more flexible and imperative DSL (Domain Specific Language) that allows users to write build scripts with extensive customization.
4. 4. Build Caching: When it comes to build caching, Buck and Gradle have different mechanisms. Buck has built-in support for caching build artifacts, dependencies, and intermediate build results. This helps to significantly reduce the build time by reusing previously built components. Gradle, on the other hand, requires additional plugins or configuration to enable build caching.
5. 5. Dependency Management: Buck and Gradle also vary in their approach to dependency management. Buck uses a more explicit and strict mechanism, where dependencies are explicitly declared in the build configuration files. Gradle, on the other hand, provides a more sophisticated and flexible dependency management system, including transitive dependency resolution and support for multiple repositories.
6. 6. Parallel Execution: Lastly, Buck and Gradle differ in their parallel execution capabilities. Buck is known for its efficient parallelization, where it can execute multiple tasks in parallel, leveraging the dependency graph and avoiding unnecessary sequential steps. Gradle, although it also supports parallel execution, may not utilize it as optimally as Buck due to its more flexible and complex build model.

In Summary, Buck and Gradle differ in language support, build speed, configuration style, build caching, dependency management, and parallel execution.