I also have the problem with http://files.minecraftforge.net/maven/net/minecraftforge/gradle/ForgeGradle/2.3-SNAPSHOT/ForgeGradle-2.3-20180601.071940-30.jar
It also gives a wrong Content-Length - in fact the same as mentioned above.
When downloading the file via browser it aborts too, but the file can be renamed and actually seems to be valid.
I guess this can easily fixed be restarting/repairing nginx, but if not, where does gradle save those files ?
Can it be tricked by placing these downloads in a specific local directory ?