I'll share the steps on how to add the customer GSTN on your invoices. Let's go to your invoice template to see it.
Here's how:
Go to Gear icon at the top, then choose Custom form styles.
Select the Edit link on your invoice template, then the Content tab.
Scroll down towards the Display section, then put a check mark next to the Customer GSTIN box.
Click the OK button.
After enabling customer GSTIN, go back to creating an invoice and hit the Print or Preview button so you can view your customers GSTIN info. Need more details? Check out this guide: Customise invoice for GST.