i am wanting to change the a2 front sight to a flip up sight, i have seen the yhm gas block flip up but its not pinned to the barrel so is this something to be concerned about, also any suggestions would help. maybe put a OR gasblock on or some other gas block that will look good.

I changed my M&P15OR to use the YHM Hooded Flip Up Front Sight/Gas Block with Rail. I liked the sight, and wanted just enough bottom rail for mounting a bipod. I'm very happy with it.
Yes, it just bolts on. Seems quite solid to me. A little thread locking compound, and I don't worry about it. If I was taking it into battle, I might insist on a pinned.
I'm no expert, but the YHM gb/fs are great IMHO.

I have the YHM folding front sight/gas block. I have not had any issues with it so far. I like it much better than the stand FSB. It really clears up the sight picture for using optics of any kind.

can u guys post pics of ur yhm sight, i was looking at the one with a rail on the bottom, also what could u mount on the rail without affecting the sight itself, such as bipod would put stress on the sight itself.

Once it's mounted it is solid. I doubt that attaching a bipod there would have any impact on the sight. At least I don't see it causing the front sight block to come loose.