Hi Greg, I have a couple bugs to report.
We use Harder Underground on our server and GregTech is slightly incompatible with it.
Similar to Underground Biome Constructs, Harder Underground replaces all vanilla stone and cobblestone (except for certain world-gen decorations like villages) with its own stone / cobblestone variants during worldgen. Its own stone/cobblestone is oredict compatible with vanilla stone/cobblestone (i.e. they belong to stone,stoneSmooth/cobblestone,stoneCobble), but right-clicking these with a GregTech hammer doesn't allow you to prospect for nearby ores with them. Furthermore, throwing harder underground stone into a crucible just instantly vaporizes it instead of allowing you to smelt it down into stone plates for instance.
Another feature that mod adds is making falling blocks "fall" sideways as well as vertically (so blocks like sand/gravel form diamond-shaped pyramids instead of pillars as they fall, making cave-ins much more dangerous). In earlier versions of GregTech 6, sand ores and gravel ores were affected by this as well (as desired), but in more recent versions this no longer seems to be the case (I believe because GregTech uses its own falling block code now?). Would it be possible to fix whatever broke the falling sideways behavior for your own blocks, or re-implement it and make it a config option?