Thanks for reaching out. This article will take you through the steps to create a transaction with 100% GST.
What I recommend is taking a look at the steps for Goods, e.g if you are wanting to record a GST amount of $100:
You will see The GST amount $100 in the bottom corner of the transactions. This will also be added to your BAS (where applicable) via the GST code chosen.
Please let me know if there is anything else I can help with?