When you've added your customer's GST number in their profile, you can display it while previewing or printing their transactions. However, you're unable to include one when exporting your list to an Excel file. You'll have to manually include or enter them into your exported file. For the complete list of customer's GST number, you can pull up the Customer Contact List report, then customise it. Here's how:
Go to the Reports menu.
Type Customer in the search menu, then select Customer Contact List.
Choose the Gear icon above the report.
Select the GSTIN box.
Click anywhere on the screen to view the updated report.
I've attached a screenshot below that shows the third and fourth steps.
After that, manually include the details into your exported list of transactions.
You can also directly pull up the Transaction List by Customer report to include the GSTIN number of your customer. Same as the process above, you can select the Gear icon above the said report and select the GSTIN option. Once done, select the Export icon, then choose Export to Excel. (Please see the attached screenshot below).
You're able to customise your reports in QBO to get the most of your transactions and focus on the details that matter the most. Then you can memorise them to save their current customisation settings. For the step-by-step guide, you can check out this article: Memorise reports in QuickBooks Online. It also includes information about exporting, editing, and deleting a memorised report.
Please know that you're always welcome to comment below if you have other concerns or follow-up inquiries about monitoring your sales transactions and pulling up reports in QBO. I'm just around to help. Take care always.