Gauge is not important - you can use any weight yarn and an appropriate sized hook!
EDIT: Please note that Round 7 of the snail shell pattern should read “sc1, 3sc1, sc6, 3sc1, SC5”
The last sc5 was inadvertently left off the first time I typed it up (yay for pack-rats keeping hand-written notes!). The web page pattern has been corrected.
EDIT 2 (son of Edit): both the shell and the body are worked in the round, and you work both sides of the foundation chain as such:
You chain A, B, C, D, E, then 2sc in D, sc in C and B, 3 sc in A, then go to the other side and sc in B and C, and put another sc in D. 10 stitches. Then it is worked around as usual.
In the body, when you get to round 9, you are working 21 stitches - this does not increase the number of stitches around (the body remains a tube of 14 stitches) but moves the starting point of the round to the other side. So round 9 is, in effect, all of round 9 plus half of what would be round 10.