NEEDLE GAUGE refers to the size of the bore or hole in the needle. The higher the gauge, the thinner the needle (and the smaller the hole). A 27 gauge needle (abbreviated 27G) is therefore thinner than a 25 gauge needle, which is in turn thinner than an 18 gauge needle. 

NEEDLE LENGTH. Insulin needles are typically 1/2 inch in length or 5/8 of an inch in length. As inscribed on packaging, needle length appears after the gauge number: "27G 1/2" refers to a 27 gauge needle that is 1/2 inch long.