Would like to find out would a US 9 in bamboo, give you the same gage on a metal, acrylic, or rosewood US 9 needle?
I expect the gauge to be practically the same but the different kinds of needles "grab" the yarn to different degrees. Metal tends to be very slick and bamboo tends to hold onto the yarn a bit more and this may affect your gauge. Try out the various needles if you can at a yarn store if you can.
My gauge on bamboo needles has more stitches per inch than my gauge on metal. On mine it ends up being a very noticeable difference. I started a scarf on babmoo and finished on metal and there was a distinct line on mine where I changed needles that you could see that I had gotten a much looser gauge on. Blocking did fix some of it, but it was noticeable.
Some people knit looser or tighter on different materials, so it may or may not change for you. The difference would certainly be less than using a size up or down.
thanks to everyone that replied. I went n bought a set of rosewood and I'm getting the same gauge as if I was knitting with the metal ones.