If you want to display your company's GST number on invoices, you can set it from the Settings > Company > Information screen in the Tax ID field. You also need to make sure that you have checked the option Show company Tax ID on invoices under Settings > Billing & Invoices > Invoice Settings in order for the Tax ID to be displayed on invoices. Additionally, you can customize the label for the Tax ID field in Settings > Display & Formatting > Custom Labels, under Company (say GST in this case). Check CORE Help Center for details.
CORE Help Center