Originally Posted by Often1

The first agent's use of the words, "no show on your first flight" suggests to me that OP, for whatever reason was not properly marked as boarding his xGRB flight and thus, as a no show, his omward flight was cancelled. That is consistent with the fact that OP was removed from his flight while still at his arrival gate (although it could have occurred at T-15).

For this reason, the issue cannot be dealt with on the phone as it requires research. Sending in a webform which is not one word over 3 short declarative sentences is your best approach. Skup every irrelevant detail such as the issue of when the aircraft landed vs. arrived and why. Skip your conversation with the GA at the arrival gate. None of it matters. The sole issue is that you misconnected and as an IRROPS passenger should have been rebooked space available in any available seat on the next flight (and waitlisted for F as a paid F passenger). As such you chose self help and now wish a refund of the value of the segment you did not fly.

You may encounter pushback, depending on what you told the AA agent when you were offered standby only. Did you expressly ask AA to cancel that segment?

Hopefully this will not befall you again, but if it does:
1. The first agent's advice was spot on. Go to the departure gate if even remotely possible. What you saw on your app, what the timings show and so on are not relevant. Flights often are held at the last second for any number of hundreds of reasons and, if there was space (seat assignment or not), a GA at departure might have placed you on the flight. If not, that agent was the agent to handle rebooking. It is entirely possible that the inbound GA placed a PNR note advising that you had been sent to the departure gate.
2. Calling it was a mistake, although there is nothing inherently wrong with it. Your ticket was loused up and it really needed an agent at the airport to fix it. Had I received the message you received over the phone, I would have politely hung up and walked over to an AC, customer service desk, departure gate or whatever and had this sorted.