After an endless number of attempts to revive Forge 1.6.4, I came across this repository, which obviously contains modified and fixed versions that can (in theory) download the missing files. However, I have this error happening.

This specific error is caused by the version of Java you're running Gradle with not having an up-to-date CA database.

I would suggest using Cracked Polished Blackstone Bricks Voldeloom nowadays rather than trying to resurrect antique ForgeGradle though. Ears' project setup is a little old.

It all worked out for me. Thanks so much for the advice!