I've not seen it done quite like this before. I like it!
It might be a bit cleaner with something like nohup $you_command_and_arguments_here 2>/tmp/stderr 1>/tmp/stdout to avoid the overhead of all the ptrace's that go on in gdb. Also, disown's implementation is defined to be shell-specific (in other words, do what works for your shell), whereas nohup uses system-level commands to abandon it's parent, so you are less likely to have strange side-effects if you try it on a non-bash shell.
nohup $you_command_and_arguments_here 2>/tmp/stderr 1>/tmp/stdout
Yeah it's definitely cleaner to do it that way, if you're organized :) I would make it even a simpler and just run it in screen or tmux. Though I know I regularly start long-running tasks before realizing how long they are going to take, so this is a quick hack to get around it.
Thomas, I think you missed the point of the gist. This makes sense if you have a process already running and later realize you need to disown it without losing its output. I for one run into this problem often and I really like this solution.
This was linked on Hacker News and in the comments there julian37 pointed out that someone created a bash script to do this called "Dupx".
Edit: And of course, thank you to bringing this tool to my attention. Very useful.