I also had a thought that it might be worth looking for little dress patterns for 8 inch baby dolls. I know that baby dolls are often rather fat and a premie is going to be anything but - but it's worth a try.
I don't honestly think a lot of extra length in the gown is going to matter because it can be tucked round Baby. But instead of the 8 inch given in the gown I'd probably knit the skirt of the gown to about 5-6 inches at the most for an 8 inch long child. Remember the head is probably going to be about a quarter of the length of the body, maybe a little more. Arms will be maybe be about the length of the torso, which probably accounts for about 1/3 of the overall length of the body. My 12 inch baby Sashas have 8 inch tummies and they're human proportioned dolls, so at a guess this child will maybe have a chest of 6 inches at the most and probably less. So a 2-3 ply yarn on 2 3/4 mm knitting needles for 2 ply or 3 mm knitting needles for 3 ply will probably be all right.
Oh gosh I'm thinking just how tiny this little baby is the poor little thing. I was a month premature and weighed four and a half pounds at birth - and my first dresses fit my 16 inch child proportioned Sasha dolls.