According to this tweet that is attributed to Muslim Brotherhood (MB) in Egypt (Not sure if they represent the real MB as I understand they were outlawed by the Sisi-Led government), the sniper rifle was locally produced in Gaza 100%. That means the charge that it was provided by Qatar to Hamas is most likely false.
In this article in Washington Post (
https://www.washingtonpost.com/news...thing-but-expert-says/?utm_term=.697ac2069334), they're claiming the sniper rifle used by Hamas to be "Austrian-made."
“ 'Hamas’ claim to have indigenously manufactured this rifle is totally false — from what is visible on the video, it’s clearly an Austrian Steyr HS.50 rifle,' Charles Lister, a visiting fellow at Brookings Doha Center who tracks weapons use in the region, said in an e-mail."
So since the first victim of the war is the truth, we may never know who provided it; and with absence of proof showing the the origins of the sniper rifle in Gaza, one would have to accept the Hamas version that it was locally produced in Gaza. It is up to Hamas' detractors to show proof that the sniper rifle came from other sources.