Hi Lucagu, 

In this case you won't be able to categorise the GST portion directly to GST Liability Payable however the system will do this for you. You will just need to enter the Expense category. 

Once you have the correct GST amount entered (after using the workaround above) and selected the applicable tax codes, a transaction journal will be automatically entered applying the GST portion to your next BAS. 

Please let me know if you have any other questions at all. 

Thanks, 

Rebecca