Haven't played with this on the Meraki Go hardware, so I may be way off-base. They may have similar terminology and setup for Meraki Go. Hope this helps.
However on Meraki enterprise you have the option of Hub / Spoke VPN. 1 site is the hub, the other are spokes and send the data to the hub. Within this the is a setting for "default route". This would funnel all traffic from the "spoke" to the "hub" then out. This sounds like what you are asking for. The other option within this is a "split tunnel" (unchecking default route). This would only send data destined for the hub down the VPN tunnel, all other traffic is sent out the wan interface (not down the tunnel)