if i run gksudo and select user and enter program's command (file name) or if i run command with gksudo -u username command or same with sudo, it does not work, only with console programs (i tried mc and even it has given permission messages.)
(this would be useful for example to run some untrustable programs as "in sandbox" i think, not giving them access to user's files.)
i have found a solution, "new session in a window" (it should be enabled in menu editor in applications/system (and installed before that, maybe)) but it runs whole new desktop session.
it is possible to run programs as root (with "gksudo commandofprogram" in "run application" applet for example) so i think may be it is already possible with some tweaks to run them as other user, i have found some information about that, and tried, but it have not worked.
as ubuntu is tweaked to run programs as root may be it is not hard to make it also possible to run as other users (simply with gksudo or gksudo -u).