Quest 803 (Gallium Arsenide) requires Arsenic Dust, which is obtained by centrifuging Realgar. But there is no recipe in the game for a sieve to collect Realgar. I manually added to the drop list for sifting diorite on the t1 mesh. In the ./config/gregtech/sieve_drops.json file, I added a new entry for Realgar, "zeolite": {"meshlevel": 1, "chance": 0.01},. This does the job, but it may cause some unbalance.

(The hotfix is just a more thorough quest description. 7ff6190 now lists cobaltite as a source of arsenic. The lack of realgar was working as intended.)

Thanks for the report, fixed it in dev as of 148cea3. Expect an hotfix to release soon