Use Android phone as Internet gateway (Not WiFi hotspot)

Question

Just wondering if it is possible to setup Android 4 phone as internet gateway without enabling WiFi tethering?

Use case:

1. Connect the phone to my WiFi home network.
2. Set phone in gateway mode (ipv4 forwarding?)
3. On any other PC in my home network set default gateway to phone IP

In theory Android at this stage would handle outgoing connections for that particular PC.

On the grand scale what I'm thinking about is this:

I have multiple phones that have unlimited 4g plans on them, every time I come back home, phone joins the WiFi network and starts acting as gateway. On my main routing box (pfSense) I setup multiple gateways (for each phone) and a load balancer. So every time phone appears, gateway changes to UP state in pfSense -> Load Balancing via that phone as well. When I leave my house, pfSense detects that gateway is unreachable -> removes it from list of load balancers.