Read from the Tablet of Theka to learn the name of the Witherbark spider god, then return to Master Gadrin.
The Witherbarks worship a spider god, and from her they collect their most potent venom. So if you want the venom, then you must summon their god!
To do that, you must know her true name, and that name is on a tablet buried in Tanaris, in the troll city of Zul'Farrak, in a tomb guarded by Theka the Martyr.
Defeat Theka and find and read his tablet to discover the spider god's name. When you know it, return to me.