Depending on the airport, they seem to be taking walk-ins too, even if the website says appointments only.
I got my parents appointments at the Miami airport in January, there were a lot of appointment availability at that time. I think they release them in batches.
In any case, while we were there, a lot of people would stop in, some had appointments and they would get next availability, but there were a lot of walk-ins that came in too.
Some obviously flew in because they had their luggage in tow but unless they asked for their boarding passes or looked at their luggage tags, there was no way of knowing if they flew in internationally or domestically as the Global Entry room was outside security at MIA, across from the Lufthansa check-in counters. There were some walk-ins with no luggage at all. They did not ask for proof of anything. You were either on the list of people with appointments or you were a walk-in.
Everyone who walked in, put their name on the list with a guy checking people in. Whenever, there was a lull between people with appointments, he would pull people off the walk-in list.
The process was super fast. My parents waited less than 5 minutes. There were at least 6+ walk-ins on top of those with appointments while I was waiting for them. After their appointment, they needed to use the rest room. By the time they finished and we walked past the room again, everyone who had been on the walk in wait list had been processed. The last couple I saw signing in as a walk-in, was exiting the interview room at that time.
If you have time to make the trip and to try to just walk-in, it is worth a shot, IMO.