I would like to use Lopy as Single channel Lorawan Gateway.
Does it also support sending downlink messages to the node? Or it only support getting uplink messages from node?
Yes! You can do both uplink and downlink messages with the LoPy as a LoRaWAN nano-gateway. You'll see that in the examples, within the docs, there is a method for the device to connect via OTAA. This method of connection requires the gateway to send an ACK, message back to the device, confirming that it's connected. This is a form of downlink message!
If you're using the Things Network, you can set up your downlink messages using their backend platform.