GuavaJS-UMD is a fork of GuavaJS that was created for postcss-rename to use in place of GuavaJS.
This is because GuavaJS 1.1.1, the current version at time of writing, isn't available on npm, and GuavaJS 1.1.1 isn't designed for a CommonJS environment.
Wrapping GuavaJS as a UMD module was chosen as the approach to resolve the latter problem.
You are welcome to use GuavaJS-UMD should you find it useful, but please be aware this fork doesn't intend to accept new features nor fix bugs in the GuavaJS library. (It would be in scope to patch upstream changes, however.)
This is not an officially supported Google product.