I'm logged on user "a" (with very few permissions), on the same system
exists user "b" (in sudo group).
When I run pkexec --user root mousepad (for example), it prompts for
user "b" password, instead of root password.
If I remove user "b" from sudo group, the above example will prompt for
root password.
I would like to keep user "b" in sudo group, and enter root user's
password.  I believe gksu used to do this.  Is this possible to do any
more in devuan?

In beowulf, I was allowed to run "su -c mousepad" but now that fails:
(mousepad:3105): Mousepad-CRITICAL **: 13:59:58.624: Failed to
initialize xfconf: The connection is closed
which is why I am now using pkexec

Actually I'll probably switch to a text-based editor for this particular
case, but in general, for GUI applications, how is this done now?

(another example, same situation)
running "gparted" from user "a" will prompt for sudo user "b" password,
instead of root password.
I seem to remember a gparted-pkexec, but don't see that in Chimaera or
Beowulf.  Also package gksu is not available.