gradlew file is a shell script that must execute during the build. When the Capacitor Android platform is added on a Windows system this file can sometimes be missing the necessary linux style execute permissions.
Permissions can be added running chmod from the root of the project.
chmod +x ./android/gradlew
If your OS does not include the chmod command permissions can be added using git.
[10:01:50]: fastlane finished with errors /usr/local/bin/fastlane: [!] Permission denied - /builds/project-0/android/gradlew (Errno::EACCES)