I first reported it on the GT5 github here, complete with crashlog - Blood-Asp/GT5-Unofficial#731

They immediately closed it and said it was on AbyssalCraft's end, so I'm reporting it here instead.

I'd say it's GT's fault for adding meta values surpassing the 16 limit vanilla has in place for blocks, but since they're not gonna change that, I'll just have my code ignore meta values above the allowed bounds.

It isnt GT issue. Item Damage and Block Metadata are different things. It's related both to AbyssalCraft and to BOP. Ore item damage doesnt equal to ore in world metadata. It's two different things, understanding them as same isnt always right. So it's ususual case when item damage of ItemBlock is used not as block metadata. And again, IT ISNT GREGTECH ISSUE!
I'm pretty sure that AbyssalCraft uses some code like that:
IBlockState blockState = ((ItemBlock) itemStack.getItem()).getBlock().getStateFromMeta(itemStack.getItemDamage); world.setBlockState(pos, blockState);
The code above used in BOP and causes it to crash with GT.
Somethink same should exists in AbyssalCraft because crash reports are same.

Apart from the world.setBlockState() part, pretty much the same code (since the crash occurs while attempting to fetch the state). I'm adjusting my code to be in par with the BoP code (since they applied a fix that stopped the crashing). If it continues after that version has been released, there's not much more to do on my end (unless I add a metadata threshold on what blocks it will try to fetch a state from). Hopefully, that won't be the case, and everything will work like it should after I've done the change on my end.