How can I force gradle to redownload dependencies?

Generally, you can refresh dependencies in your cache with the command line option --refresh-dependencies. You can also delete the cached files under ~/.gradle/caches. With the next build Gradle would attempt to download them again.

What is your specific use case? Do you use dynamic dependency versions or SNAPSHOT versions?

On Unix systems, you can delete all the existing artifacts (artifacts and metadata) Gradle has downloaded using:

rm -rf $HOME/.gradle/caches/

Note that --refresh-dependencies won't always re-download every artifact; it will use existing copies if they match what exists in the repository. From the Gradle User Guide, refreshing dependencies:

The --refresh-dependencies option tells Gradle to ignore all cached entries for resolved modules and artifacts. A fresh resolve will be performed against all configured repositories, with dynamic versions recalculated, modules refreshed, and artifacts downloaded. However, where possible Gradle will check if the previously downloaded artifacts are valid before downloading again. This is done by comparing published SHA1 values in the repository with the SHA1 values for existing downloaded artifacts.

[...]

It’s a common misconception to think that using --refresh-dependencies will force download of dependencies. This is not the case: Gradle will only perform what is strictly required to refresh the dynamic dependencies. This may involve downloading new listing or metadata files, or even artifacts, but if nothing changed, the impact is minimal.

If you are using a recent version of Gradle, you can use --refresh-dependencies option.

./gradlew build --refresh-dependencies

you can refer to the Gradle manual.

The --refresh-dependencies option tells Gradle to ignore all cached entries for resolved modules and artifacts. A fresh resolve will be performed against all configured repositories, with dynamic versions recalculated, modules refreshed, and artifacts downloaded.