Yes it has 100% to do with GalaxyAdditions, the Exception literally comes from that Mod. Please Report it to that Mod as I cannot help you with something that neither comes from Thermal Expansion nor GregTech, even if the connection of those two Mods somehow triggers it, it is still GalaxyAdditions Code that is at fault for this crashing in the first place.
Edit: I can even tell you why this happens, GalaxyAdditions somehow tries to access Textures while being on the Server even though Textures do not exist on Multiplayer Servers at all. That is quite literally where the crash came from and I have no Idea why the crash would happen in this weird Situation, but there must be some exceptionally terrible Code on GalaxyAdditions Side for this to be caused when GregTech and Thermal Expansion Blocks interact.