Agent allocations are a way to record the contribution an agent made to the sale of a listing.
Creating agent allocations by entering 'including GST' figures implies that all agents receiving an allocation are registered for GST, however depending on which agents you select to be given an allocation this may or may not be the case. For this reason it is not possible to enter agent allocations using 'including GST' figures.
If an agent is registered for GST, enter their allocation 'excluding GST' and the GST will be added on in that agent's commission allocation section.