Sounds like an item ID conflict. Try changing the item IDs in the appropriate GregTech config file (Unleashed/minecraft/config/GregTech/BlockItemIds.cfg or similar) to a set of IDs that don't overlap with the IDs in the BoP config.
Also, if you create a world WITHOUT GregTech, install GT, then try to re-open the world, you should get a screen warning you that some IDs have changed.