What's the pronunciation of this character？ 匄 gai4 or 匈 xiong1？

Good question. The original evidence does not provide pronunciation. Based on the evidence 2 which gives ⿰木匄 and which is also one of the earliest historical evidences, my educated guess is that the modern pronunciation could be 匄 gai4.

Note that we normalize ⿰木匄 to ⿰木匃 per UCV #143.

The glyph is set to ⿰木匃 because there are already encoded characters where 匃 is the right component: 𦍨𩢛𭠝𮌍𮠤, while no encoded characters features 匄 as the right component.