After upgrade Gregtech mod from 5.09.27 to 5.09.28 server has crash with java.lang.OutOfMemoryError: Java heap space when player try to connect (from 3500M with 0 players up to 6000M with one player)
crash-2017-03-06_11.18.34-server.txt
With Gregtech 5.0.27 about 3500M of memory in use all time with same mods and no issues.

Issue was fixed in Blood-Asp/GT5-Unofficial#931

in config/CoroUtil.cfg set chunkCacheOverworldOnly to true to prevent this from happening.