Good question! The main difference is how access is managed. Instead of configuring ACLs, you define policies which are a 1:1 mapping between a user group (manually created or synced from your IdP) and the resource you want to allow access for. Another difference is how our load balancing / failover system works - it's automatic across all the Gateways in a particular Site.
For simple access needs, in Firezone you would likely configure a CIDR resource and grant the Everyone group access to it, which mimics the setup of a traditional VPN. It is a couple extra clicks, though.