Hi Melissa,
When in a purchase transaction you can just type in the GST code and it will let you use it.
Or you can go to the Tax Item list and create a tax item called GST-P, then edit the tax code GST to include the GST-P tax item.
But as Charles mentioned this can affect your tax reporting and thus may be wrong.
If you would like a document on how to use them correctly, feel free to email me directly.
Kind regards,
Sally McIntosh (sally@samsolutions.com.au)