With Gregtech 5 installed, I'm pretty sure the only way to get U-235 is to centrifuge uranium-238 Dust. You get a 20% chance to get a tiny pile of U-235. It costs ~230,000 energy though in a centrifuge. (Note centrifuge, not thermal centrifuge). There may be U-235 ore, but I haven't found it yet. I haven't done much quarrying though
I checked the metadata values via NEI (and NEI Integration). Uranium 235 ore has metadata 97, but the default WorldGeneration.cfg file uses metadata 98 (Uranium 238 ) for the Uranium ores in both Plutonium veins and Pitchblende veins. However, due to the existence of Plutonium ore in GregTech, I think you can actually make MOX fuel more easily than Uranium fuel. If you're using the 5.08 version of GregTech that Blood Asp made, you even have the option of making thorium fuel rods.