You can obtain Obsidian Gypsum by defeating level 60+ open world named bosses, typically found in Elite Landmark.
Drop Quantity Limit: 3
Time Between Drops: 5m - 6m 30s
Limit Expiration: 18h
Note: the drop counts towards the limit as soon as it is on the ground (when a loot bag drops). If you do not pick it up, you will miss one gypsum until the limit expires.