Looks like a Mateba Unica 6 to me, best visible at 0:12. Any firearm experts feel free to correct.
The creator replied to their tweet with the gun model they used for this, apparently it's a Mateba M-M2007(featured in the first two GitS films), which is not a real gun, but its closest real-world counterpart is the Mateba 2006M.
Looking through our existing Mateba tags, it seems like we've got a few guns incorrectly tagged Mateba_2008M when they should be Mateba M-M2007 anyway (an easy way to tell the difference in this case is the cylinder, which is fully housed within the frame of the M-M2007(and 2006M), where the M2008(and model 6 unica) has an open-top housing.), so I'll add the tag.
