The Syrian conflict is not very straight forward and claims of chemical attacks should be treated with hessitation as they could spark a regional conflict.
Since the recent forced evacuation of various rebel groups from Eastern Ghouta, media outlets have been reporting discoveries of chemical weapon manufacturing bunkers underground, primarily in the last current bastion of rebel held Ghouta.
The refusal of the group to relocate may add an element of truth to the unconfirmed discoveries. It is further solidified when you take into account that the same rebel group previously used chemical agents against syrian population in Aleppo.
The recent targeted strikes by syrian and russian aircraft *may* have hit whatever stockpile was co-located with the rebel groups.
Tactically and strategically, it is unlikely that the Syrian forces used chemical weapons against the rebels and civilians in Ghouta. Especially at this late stage of the conflict while relocation deals are being handed out.
It is more likely that remaining rebels used the chemical agent as a last attempt to regain control and halt syrian advances by bringing the international community into the mix.
Thats IF there is any truth to it.