Re: vxlan L3 gateway best way to reach rest of the network?
Do you mean the clients within the DC to reach the outside? Then you just host the clients behind the leafs and use any protocol (OSPF will do the job) and redistribute any outside routes (or simply default route) from PE on your gateway (Spine). On the Spines, need a routing-instance where you advertise the L3 interfaces (say IRBs?) for connectivity to the end devices of the DC.
Assuming any other outside IP clients that need connectivity to the DC have connectivity to your PE, above should get you connectivity to such clients into the DC and vice-versa.