Presumably only one of the NICs is connected to a network which has Internet access?

The GUI way is to go into TCP/IPv4 Properties, Advanced, and set a default gateway with a metric lower than any other default gateways (Windows will prefer the default gateway with the lowest metric). If you're still a bit stuck, do a "route print" and show us/me - via Hangouts if you want it less public.

If your machine is configured to use a DNS server on one of the local LANs, you'll need to make sure it's reachable.

I'd say get rid of the DFGW completely on the second NIC. Even with metrics, it doesn't really make much sense to have two gateways defined IMHO - should just add static routes if required.