There's no Form CG1 for 2013 yet, because you can't file a CGT return until after the year ends (i.e. you file for the whole year).
What you
are obliged to do is pay the CGT by the due date for payment.
But to answer your questions, for when they do print the 2013 form:
7. Chargeable gains - this is the amount of the "Gross gain" as you call it (by which I presume you mean the difference between purchase and sale price), LESS any expenses, indexation etc that you are entitled to deduct.
12. This is the chargeable gain from 7. as reduced by losses carried forward and the annual exemption.
15. & 16. should total to the same figure as 12 & 13 (in most cases 13 is nil) - 15 relates to the first part of the tax year, and 16 relates to the second
You don't need to state on the form the amount of actual tax due - they compute that based on what you've filled in, and it should match your payment(s) for that year, which are due many months in advance of the return itself.
Form CGT2 should help you to digest the important points regarding the separate obligations of payment & filing returns
http://www.revenue.ie/en/tax/cgt/leaflets/cgt2.pdf