For those who have tried to complete the quest using snake venom sacs, Gindlin will return them with a whatever comment. However if you give him 2 spider venom sacs as requested, along with Crow's Special Brew (I purchased mine from Crow in NQ) and 20 gold, you do indeed get a vial of spider venom.
[Mon Jan 11 19:16:21 2016] You say, 'Hail, Gindlin Toxfodder'
[Mon Jan 11 19:16:21 2016] Gindlin Toxfodder says 'Enough with the formalities. I don't make it to these parts often and I only plan on being here a couple days. My time is precious. You must be a rogue in search of the [death juice].'
[Mon Jan 11 19:16:23 2016] You say, 'death juice'
[Mon Jan 11 19:16:24 2016] Gindlin Toxfodder says 'Spider venom. You can't mix it yourself. If you want, I can do it. I'll need two spider venom sacs from the giant spiders. There should be some around these plains. I also demand a fee of 20 gold pieces, oh yeah!... I have been banished from Qeynos... I also want a bottle of my favorite brew, Crow's special brew, purchased from Crow himself in North Qeynos.'
[Mon Jan 11 19:16:47 2016] You say, 'Spider Venom Sac x 2'
[Mon Jan 11 19:17:00 2016] Gindlin Toxfodder says 'You did hear me, didn't you? I want two spider venom sacs, a bottle of Crow's special brew and 20 gold pieces.'
[Mon Jan 11 19:17:00 2016] Gindlin Toxfodder says 'You did hear me, didn't you? I want two spider venom sacs, a bottle of Crow's special brew and 20 gold pieces.'
[Mon Jan 11 19:17:00 2016] Gindlin Toxfodder says 'You did hear me, didn't you? I want two spider venom sacs, a bottle of Crow's special brew and 20 gold pieces.'
[Mon Jan 11 19:17:00 2016] Gindlin Toxfodder says 'Here. I could care less what you do with this. Hopefully you'll lay some on the Circle of Unseen Hands.'
[Mon Jan 11 19:17:00 2016] You gain party experience!!
[Mon Jan 11 19:17:00 2016] You gain party experience!!
[Mon Jan 11 19:22:51 2016] Gindlin Toxfodder regards you indifferently -- what would you like your tombstone to say?