I've found this article from ATO's website which provide a comprehensive information on completing your BAS for GST. On the other hand, you may consult an accountant or tax agent to ensure accuracy on your tax reporting.
If you don't have an accountant yet, you can find one near you through this link: https://quickbooks.intuit.com/au/find-a-proadvisor/.
Let me know if you have any other questions and I'd be glad to help. Have a great day!
Thanks for the reply @IntuitAika.
I'm expecting that this should be a simple yes or no answer from Intuit that should be consistent with all instances of the software. If Quickbooks is providing the figures for 1A and 1B, then it must also know the answer to question G1, since G1 relates directly to 1A and 1B.
The G1 amount is summed up from the total GST of all Sales transactions that have GST on the transaction. Make sure to check your transactions and see if they're coded with GST. If you are missing some of the G1 amounts, you'll have to find the transactions that are missing and include GST in them or recreate the missing transactions to increase or show the G1 number.
You can run the BAS Summary report to check the details. Here's how:
You can also run a Transaction by tax code in the GST center. You can check this link to learn more about Simpler GST FAQ.
Feel free to comment below if you need more help with QuickBooks Online.