With a G-account, you give your client certainty that you will pay payroll taxes and/or VAT to the tax authorities.
The G-account agreement is the proof that you have such a G-account. Upload it into Bouwpas so that your client can approve it.
You can open a G-account with all major banks. You also must apply for the G-account with the tax authorities. .
You can find the form here: Link