They had the means constructed and had produced Military Weapons Grade chemical weapons and had those weapons already in their possession in the field.
Yes, but there was more to it: the Ghouta enclave was broken into three parts about a month ago, and similar chemical facilities were found in the parts liberated earlier, the Douma enclave -- where the alleged chemical attack took place -- was the last to be freed.
Right now the entire Ghouta is free from the Islamist forces, meaning that no more shelling of Damascus and no immediate threat to the capital.