As noted above, pluma works correctly in parentheses. Remind does not. At least, the syntax...
(remind -z0 -k'gxmessage -title "reminder" %s &' ~/.reminders &)
fails, as do a couple of variations on the poisition of the closing parenthesis. Putting the & after the parenthesis closes the app when the terminal is closed. Without the & the same applies, as is expected.
disown in front of the remind command with parentheses returns syntax error, without it returns "no such job" for several parts of the command. Removing the trailing & makes no difference.
nohup declares it is ignoring input and the output will be sent to nohup.out (which was last created/updated several days ago and not since). Still no sign of remind in System Monitor.
I've also tried removing the first & but that makes no different to remind remaining open when terminal is closed.
Obviously remind works for some people but I can't get it working without leaving terminal open. :(