A dupe of my post (almost exactly).
#justsayin
I've modified my post subject to include "Could not find com.mojang:authlib:1.5.16" to (hopefully) cut down on further dupes.
Not 100% sure what's wrong where, but no matter what gradlew operations I'm trying this morning, they are all failing:-
Something wrong at my end, or is it (as it naively appears to be) a file missing on the 'forge servers?
(Apologies if this is in the wrong place, but the ForgeGradle thread says post support here...)