I don't know what kind of proof you require pfr but I do know that many agencies including the OPCW have confirmed well over seventy chemical weapon attacks in Syria with some having been delivered by plane or helicopter. Seeing as how none of the groups fighting Assad have planes or helicopters there is only one logical possibility as to who is the guilty part unless you're some kind of conspiracy nut.

The Independent International Commission of Inquiry on the Syrian Arab Republic says it has confirmed at least 34 chemical attacks since 2013, many of which it said used chlorine or sarin, a nerve agent, and were conducted by the Syrian government.

Human Rights Watch has reported that there have been 85 chemical attacks since 2013, based on its analysis of reports verified by various sources, including United Nations investigations and Amnesty International.

Link to the NY Times article with a list (Source: Independent International Commission of Inquiry on the Syrian Arab Republic) of confirmed chemical weapon attacks and the perpetrator.