I am not on the GrapheneOS team, but as a professional, I use and recommend them both. Each have their own strengths for different purposes. It's important to have at least one backup VPN. I also recommend Mullvad, though I do disagree with the way they've handled some things recently. AFAIK GOS does not have an official VPN recommendation, but team members may have their personal preferences. We'll see what final has to say about it :)