Which modpack version are you using?
#2.0.7.5

I test these GT++ Multiblocks, all of them don't need core module, some of them with core module will show "incomplete structure"(Ore Washer, Thermal Centrifuge, Amazon Prime, Energy Station).
GTNewHorizons/GT-New-Horizons-Modpack#4877

Did GT++ cancel the core module?

This was intentional as multiblocks do not need cores after a certain point in GTNH's version history (post v6.2+?). This was done due to various complaints about it when the cores were first introduced and were made optional in the config file of GT++ (or were hardcoded to be off).