If you scroll back a few dozen posts to a
seat swap I had this summer, you'll see I that asking the GA can lead to a literal shouting match (between other pax and GA) and then a disgruntled seat meat for 8+ hours. Would have been much less disruptive if we had offered our one E+ seat to an E- pax onboard without going through UA, and several people on this thread suggested that that's exactly what we should have done (instead of asking the GA). Not sure I agree; I think asking GA (before boarding, no line at podium) if it's possible to sit together is totally reasonable; I also think it's fine to orchestrate 1- or 2-step swaps onboard, esp for long flights, as long as it's done politely, you're not trying to downgrade anyone, and you can take "no" for an answer. It's simply inefficient for people who want to sit together to sit apart when other people could sit in equivalent seats (and if you don't think the seat is equivalent, don't say yes!).
Some people say "only approach me once I'm seated; never poach my seat", while others say "please ask me before I get settled". Some people say "always go through GA/FA", other people say "I prefer to be approached directly and not involve the airline".
Some people have been burned badly by people trying to pull a fast one, so they say "no" every time. Others have been saved (able to sit next to small child or elderly parent, or maybe even their able-bodied partner) through the kindness of strangers and are happy to pay it forward.
So, it seems like there is no one-size-fits-all solution. Different passengers have different preferences. I think the conclusion I've reached from this thread is "feel free to ask GA or other pax; be prepared for a 'no'; be aware some people might think you're a jerk even if you ask politely and take the 'no' gracefully". When it comes to a situation where I can ask politely to sit next to my SO for 8 hours and the outcomes are 80% "yes", 15% "no" and no hard feelings, and 5% "no" and a total stranger thinks I'm a jerk, I'm going to risk it and go ahead and ask for the swap. Sorry, not sorry.