What does 0.0.0.0 gateway mean in routing table?

Where the gateway is all zeros, it means there is no gateway.

This is because the networks in question are directly attached to the machine, in that the machine has an IP address on an interface that falls into this network subnet.

Any packets for these networks don't need to be routed, as they are connected, so packets can be sent directly to the destination on the local network.

If the machine has a packet destined for another device on these networks, it will do an ARP request, to find the MAC address that is associated with the IP, and transmit the packet directly to that MAC address.