Welcome @mjchrist!
Short answer: Check the return value of send() on the gateway, to see if a message has been successfully delivered an outgoing message (via Ethernet, MQTT, serial).
Long answer: What you are referring to as an ACK should really be called echo instead to avoid confusion, because that is really what it does.
bool send(MyMessage &msg, bool echo);
If you set the echo parameter to true, you ask the destination node to echo the received payload back to the sending node. An echo is only being generated for messages that travel through the MySensors network. For example, if node 42 sends a message to the gateway (so that it can forward it to the controller), the gateway will echo the payload back to node 42. The same is true if the gateway is the sender and node 42 the destination.
On the other hand, the boolean return value of send() will tell you that the sending node received an acknowledgment from the next hop. This next hop might be the destination if there's a direct connection between the two nodes, or any node that relays the message further to the destination, like a repeater.
In the special case where the gateway sends a message (to the controller, so to speak), the return value of send() isn't the ACK from a MySensors-internal transport (like NRF24), but the confirmation that the message was successfully delivered via the outgoing transport layer which translates the message into a packet, a MQTT topic or string for the serial port.
Oh, and by the way, please use isEcho() in favour of isAck(). The latter is misleading (IMHO) and deprecated. It will be removed with MySensors v3.
And last but not least, MY_TRANSPORT_WAIT_READY_MS doesn't mean that the node (or gateway in this case) waits for n milliseconds until it starts establishing a connection (for the transceiver (NRF24), to be clear). Instead, it limits the time the node tries to establish a connection to n milliseconds, so that it can continue to work autonomously and do other stuff if it failed to do so.
