The gradle wrapper for rift uses the -all gradle installation, which is a lot larger than the binary distribution because of included sources and gradle documentation.

To fix this, simply run the gradle wrapper task with the following argument:
wrapper --distribution-type bin

This is intentional, so that we get the gradle sources and docs in the IDE... This way we can use Ctrl-Q to get docs for any gradle task we're using, and Ctrl-Click to jump to its source. According to your logic, we should make gradle not download any sources for any of the libraries we're using.

no, it just seems pointless to me to make everyone download the gradle sources when developing rift.
If it was for developing a gradle plugin i'd agree, but not for developing rift or rift mods..

Maybe it's pointless to you, because you don't need to edit Rift's build.gradle. I don't want to have to look up the gradle docs every time I want to do something in gradle. Pressing Ctrl-Q allows me to quickly see the documentation for any feature I need in build.gradle:

The gradle sources are downloaded just once for version 4.9 and will be reused for every other project.

figure out a way to change it yourself on your own system, but don't distrubute a build.gradle doing gradle-all. maybe make it a config option. or just commit distributionUrl=https\://services.gradle.org/distributions/gradle-4.9-bin.zip in gradlew.properties but have it -all in your personal filesystem.