If you want to enter a purchase invoice for landed costs that also includes a line amount which represents 100% GST the best method to enter the purchase invoice through screen ID AP301000 menu path [Finance \ Accounts Payable \ Workspace \ Enter \ Bills And Adjustment].
When entering the purchase invoice, the 100% GST component is required to be entered onto the documents details tab and landed costs lines can be entered onto the tab Landed Costs so that these costs can be spread as required by the landed costs code.
If you try to enter the 100% GST through the purchase receipt using a landed costs code or through an accounts payable bill again using a landed cost code within the landed costs tab then the behaviour will not be as expected, what you will notice is that the 100% GST amount will be halved.