We are using the RAK7243 to forward LoRa packets to a proprietary non-LoraWAN server.
Is there a way to get ACK packet transmitting from the GW to the end unit on every received packet it got?
The “gateway” side of a gateway never generates radio traffic, it merely passes uplink reports to the network server, and transmits downlinks only if explicitly told to do so by the network server.
Since you’re running with an external network server, only that software is in a position to tell the gateway to transmit.
But you should note that replying to every transmission quickly becomes impractical and so is discouraged. From a regulatory perspective, the gateway is one device, and only gets one allocation of airtime/duty cycle. Additionally, while a gateway can receive 8-16 uplink signals at one time, during any time when it is transmitting to one node, it cannot receive signals from any other.
To put it simply, responding to every message not only jams others, but can even jam your own network.