Meeting the exact gauge is not crucial, but any major differences will affect the size of the finished shawl.

You will always knit the first and last 3 stitches of the row, no
matter what side you are on! This will prevent the edges from
rolling.
On all right-side rows, you will increase 4 stitches per row.
On all wrong-side rows, you will only increase 2 stitches per row!
Note that, on the wrong-side rows when purling, you will always
knit the center stitch!