First time setup of High Available F5 BIGIP pair...what IP should I set servers to for default gateway/route?

If you are asking which default gateway the BigIP should use, that is entirely dependent on the rest of your network architecture. Bear in mind that you will likely want a different default gateway for your management and your tmm networks. There is no reason the two BigIPs can't use the same default gateway.

If you are asking about your Pool Members, and you want to use the BigIP as your default gateway, I would suggest that you use a floating self IP on the relevant VLAN with a wildcard forwarding virtual server. The floating self IP will follow whichever unit is active, and the forwarding virtual server will pass the traffic along based on it's networking table. Please make sure you have a good default gateway on the BigIP in this case.

The BigIP is a default deny device, so the forwarding virtual server is needed to provide a listener for the unsolicited outbound traffic from the pool members. You want the floating self IP because the Standby device will not pass traffic, and you want the default gateway because the BigIP needs to know where to send traffic when it doesn't have an explicit route for the site in question.