To start, here is a brief summary of arguments that highlight the unlikeliness of the syrian army being behind the chemical attacks around Damascus:

1. The syrian army has been making a number of military gains, especially in the Homs governorate but also in and around Damascus. The current military situation was not threatening at all. Resorting to chemical attacks in the midst of „normal“ military clashes does not make sense.
2. The syrian government for month has been threatened with military intervention by Nato countries in case the „red line“ of using chemical weapons would be crossed.
3. It has been the syrian government itself that invited a UN inspector team which arrived only 2 days before the alleged attacks.
4. Contrary to repeated accusations by western media and politicians from the US, UK and France (among others) the syrian government is not „insane“. Even after being subject to multiple attacks by the israeli airforce and artillery and in one case by the turkish army – acts that resulted in destruction and many casualties for the syrian army – the regime did not hit back, knowing well that any violent reaction would trigger a massive military attack that would be suicidal.

But there are more questions:
a) If it was the syrian army why did they not use „regular“ and much more deadly chemical weapons that they posses?
b) Why did they chose areas around Damascus for such an attack?
c) Why did the army not carry out a more massive chemical attack followed up by a major troop incursion, both to finish off the (remaining) rebel fighters and  – more important – remove the traces of the attack?
d) If it was the army, it must have been crystal clear to them that such a gas attack cannot be concealed. They would know that the victims would be brought to hospitals and clinics. It should not be difficult for the army to take control of those facilities in advance to avoid that medical reports „leak“ out and doctors give interviews that could implicate the army. Besides, the regime could break down the internet connection and block access to Youtube and co., while being prepared to spread their own (fabricated) story to the media before the „activists“ shape public opinion
e) It would be obvious to the army that the military value of apparently blind attacks on civilian areas supportive of rebels would be limited while the resulting outrage and revenge feelings would be huge

In addition there are a number of valid reasons why it is not just a conspiracy theory to suspect the rebels:
https://radioyaran.wordpress.com/2013/08/22/why-it-is-not-unlikely-that-the-rebels-could-have-used-chemical-weapons/

A good scientific analysis:http://strongpointsecurity.co.uk/site/wp-content/uploads/2013/08/Revised-Thoughts-on-Damascus.pdf