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It's most likely an ID conflict, write down the Gregtech items you're getting from them and then check the Gregtech config to see what their ID's are, cross reference this with the Biomes o' Plenty config to see if the ID's are in use there as well.
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.
If you look in the console when you start the game, ID conflicts will be prefaced with CONFLICT in big letters and say which specific IDs are in conflict. Copy the console log into a text editor and just search for all conflicts so you know which IDs to change.
For future if anyone wants the configs to fix this, here you go. Just remove the .txt extension (it wouldn't let me upload a .cfg for some reason
The main.cfg goes in the biomesoplenty folder and BlockItemIDs.cfg goes in the GregTech folder
-Just because i am not quite sure, if im not allowed to post .cfg files just reply to this and ill remove them