From your link:No. Douma was won officially on March 31. Only a small number of the terrorists remained and were in the middle of negotiating a cease fire with the Russians.
Air strikes carried out by the Syrian regime hit the town on Friday in response to rebel mortar fire from Douma that wounded seven people near the capital.
Apparently air strikes and mortars were part of the negotiations. If air strikes were still taking place, why would gas but out of the question?