Well I've never done a gran fondo. If my understanding is correct, they are different from organized rides I have done in that you are timed. But I think they are generally the similar to the rides I've done.
In general when you ride with a group, you want to be predictable and you want to communicate. Signal or say something before slowing down, stopping, moving left or right. Also signal or announce debris or obstacles on the road that people behind you need to know about, for example point down at a pothole or gravel as you approach it or say "gravel" or "pothole." If there are cars coming from behind you warn others with "car back." Cars coming toward you from the opposite direction are "car up." Be aware of what's going on around you -- if you want to move left or right, make sure there's no one behind you that might hit or cut off. Pay attention to the rear wheel of the bike in front of you -- if you touch wheels, the rider in back is likely to fall. If you pass someone, say "passing" or "on your left." If someone is passing you, hold your line. Signal before you turn.
If it's show and go -- arrive at the start, sign in at registration, get ready to ride and leave when you're ready -- things are usually pretty low key at the beginning. There may be others around you but there likely won't be a big crowd. If it's a mass start, then everyone will start at the same time. They might group you according to the distance you're riding or the speed you expect to ride and start the groups in waves, but you'll still be in a crowd. This type of start is more tricky. You probably will go very slow at first so my might want to keep one foot unclipped in case you have to stop. Then for the first few miles faster riders who started behind you will be passing you. Typically things shake out after a while and there's less passing going on. You often end up with a few people who are riding the same pace as you, so you stay together for the ride, or at least until the rest stop, which can be nice if they're friendly. Occasionally I will end up riding with strangers who are erratic or annoying in some way, so I just slow down a bit or make an excuse to stop so they get ahead of me. But that's rare.
I'm average when it comes to ride speed -- about 13ish to 14ish mph depending on terrain. For long organized rides, I often am actually one of slower riders out there. There have been times when I got caught up with the excitement and tried to hang with faster people. That's often not a good strategy because you end up exhausted before the end. Also you might see people riding in pace lines -- taking turns drafting off each other. If you're not experienced with pacelines, just hold your line and let them pass. It's best not to try to join them unless you're familiar with how they work.
Since winter is approaching, I assume the ride you're looking at is sometime next year. If there's a bike club in your area or other organized rides before the gran fondo, it might be good to check them out so you can get used to riding in a group.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles