The gateway doesn’t know the difference between LoRa and LoRaWAN 
So when one of your LoRa devices would send an uplink LoRa frame, it would publish this to gateway/[GatewayID]/event/up with the LoRa payload as phyPayload field.
LoRa Server will not know how to handle these frames (and will probably log this as an error). So you need to create your own application subscribing to gateway/[GatewayID]/event/up and filter on these “raw LoRa” frames.