Baby gags when I give breast's nipple...why?

So my nipples are inverted and no matter what i did LITERALLY they didn't help in breastfeedung my baby as the supply was good but everything got messy whenever baby tried to feed so fast forward 1 week later after his birth i started to use nipple sheilds and they have been a life saver seriously.

Now the thing is i use to atleast try to give him nipple sometimes just to see if he would take it but he didn't and thats all. Now he 3.5months old and i tried now several times but he ACTUALLY GAGS AND MAKES A VOMITING FACE WHICH HAS REALLY GOT ME CONCERNED.

Anybody else experience this? What could be the reason...?