If you properly implemented the portion of the Tutorial to set the GCN and GCB Ticket Tags, this should work to print Balance of the GC on the Ticket. If the balance is 0, then nothing will print… this is because of the [square brackets] that surround everything. To see what is contained therein, remove them. But I would guess there is something wrong with your setup.
Change this:
[<F>=
<J00>{TICKET TAG:GCN}|Balance: [=F(TN('{TICKET TAG:GCB}'))]]
To this:
<F>=
<J00>{TICKET TAG:GCN}|Balance: [=F(TN('{TICKET TAG:GCB}'))]
This is supposed to automatically use the GC as Tender without overdrawing it. It saves you needing to use a payment value first. It decides if the GC has enough funds to pay the Ticket. It will pay the whole Ticket or a portion of the Ticket, based on Ticket Total and GC Balance. Again, if it does not work for you, something is wrong is your setup.