Hello... I wanted to mount a two-point sling. It appears there is a hole on the gas block for attaching a quick detach swivel for a sling. Anyone know which one fits or can you recommend one that fits? Thanks for any help.

The size is standard. Loop size can vary depending on your preference of sling width.

Is the gas block one that is doing triple duty? serves as gas block, flip up front sight and quick attach for sling?

In my opinion I would not want the sling attached to the front sight post.

I would add the quick detach to the handguard, use whatever you have an M-lok, keyMod or picatinny rail.

I have Magpul QD on all three of mine.