Currently I have 1 defualt gateway that I route all my LAN traffic to the Internet.
Can I have a second one for failover\redundancy? How would the configuration work on my LAN machines ? Is it just a case of adding a second gateway on my devices' TCP\IP config ?
Yes to minimize the impact on users and prevent interruption it would be wise to use the existing address of the router as the shared address and to assign new addresses to the router interfaces.