How should I treat an input credit available in GST which I have not utilised in the same month?

Hi there, allan.govias. 

You can add the bank as a supplier and use it to create an expense. Here's how:

1. Select Expenses, then Suppliers.
2. Select New Supplier.
3. Complete the fields in the Supplier Information window.
4. Select Save and close.

You can follow these steps on how claim the input tax.

1. Click the + New button and select Vendors.
2. Under the Payment account, select your bank.
3. In the Payment date field, enter the date.
4. From the Category column, choose Bank Charges expense account from the drop-down list.
5. Enter the amount and fill out other necessary fields.
6. Click Save and close.

For future reference on how QuickBooks generates reports on GST, you can click this article: The GSTR3B (GSTR 3B) report in QuickBooks.

Let me know if you have other questions. Take care and have a great day!