Which brick is the gray one? I don't know which brick the gray one is. Can anyone help me?
The gray part is a 43713   "Wedge 6 x 4 Triple Inverted Curved".  It has appeared in 231 sets since it was introduced in 2003.  Your picture looks like light gray.  This part has only been in 28 sets in Light Gray or Light Bluish Gray.



I found it on bricklink by searching for "curved 6 x 4".  I also tried "curved 4 x 6", "fuselage", and "bow" (as in the bow of a boat) before finding the right search.