Unity has a default gradle.properties file that gets added during the build process. While its possible to change the build.gradle and the settings.gradle files as mentioned
In the newer Unity versions (2019.4+) it is possible to generate a custom gradle properties template by going to Project Settings > Player > (Android Tab) > Other Settings > and marking "Custom Gradle Properties Template".
After selecting that a gradleTemplate.properties file is generated at "Assets/Plugins/Android/gradleTemplate.properties".
This is the best way of generating the file since it is git friendly and preserves other settings.