Metamorphic granite doesn't exist. Granite is an igneous rock. Gromnik is probably of granite (I've been there both on foot and on my bike, or rather carrying my bike :) but I don't remember what the rock was). The Strzelin Hills are composed of both granitoid rocks and metamorphic rocks.
It reads here that in the 15th century the German name was Romsberg, which literally translates as Gromnik.
Just a question before I publish the changes, despite of my (modest) searches I have no clue what Gromnik means (something tells me it could mean fortified place but I just don't see how). In German too, I have no clue what Roms neither Rummels means...
The Polish name means a hill that attracts thunderbolts. The problem is, although the text implies Romsberg means the same as Gromnik, online dictionaries don't seem to confirm that (I don't speak German). One thing is sure: The Polish name stems from the word "grom", which translates as "thunderbolt" in English.