Receiving Items Without POs
If you would like to have a payable bill created when you purchase something from a vendor, then you need to create a PO, receive it, and then do Expenses -> Purchase Orders -> Completed PO to AP. This will create credit entries in your AP GL account.
Receiving without a PO (Expenses -> Purchase Orders -> Receive Without PO) does not create entries in the AP GL account, though it will make GL entries for inventory. AP GL account entries are only created when adding Vendor Bills (Expenses -> Vendor Bills -> New Vendor Bill or through the Completed PO to AP process).
You can change the credit account after you receive without PO in Ledger -> Journal Entries (click on the link in the Voucher column for the entry you'd like to edit). You can also make a new entry to move amounts from one account to another for the receipt without PO entry.
If you need AP tracking for inventory purchases, then create POs, receive them, and then convert the receipts to AP bills.