Is it gauche to go back to an old thread, a really old thread-
If you have an update on something or discovered an answer to a question that was never resolved when it was new?
Or are we to assume that nobody cares about the subject anymore?
A breaking update or a newly-discovered answer is a great reason to revive an old thread. Go for it.
However, bringing up an old thread just to revive the conversation (without something specific to add, or just to offer an opinion on a long-gone thread) is generally frowned upon as a matter of netiquette. For myelf, when I see an old thread popping up (especially in Comments or GQ), I expect something has something important to add. If it’s just a “me too” or a troll, I get annoyed at the waste of my time.
Did that make sense?
It’s also nice to include a comment with your post that you are indeed reviving an old thread. Sometimes it can be confusing or startling to see old members names pop up and not everyone reads the timestamp on each post.