gkb defaults to use xmodmap to set the new layout. I do believe and try to
prove now that this is a really bad idea and setxkbmap should be used by
default (i.e. setxkbmap should be offered to the users who don't know what
xmodmap or setxkbmap means).

(However, bug #108406 has to be solved first in order to successfully use   
setxkbmap with gtk+-2.2.)

When adding a new layout, the list of keyboard maps should be generated
based on the available xkb layouts, and a setxkbmap command should be
generated. When setting the properties of a particular layout, the examples
should mention setxkbmap first and xmodmap/gkb_xmmap later. If gkb wants to 
restore the startup time layout, it should execute 'setxkbmap -v' to query
that and later setxkbmap to restore that.

My arguments will follow here. But first of all I mention the fact that
KDE's keyboard applet uses setxkbmap.

gkb's homepage says that 'On a typical Linux system, xmodmap files can be
found under the directory /usr/share/xmodmap'. On my system no-one else than
gnome-applets itself ships files to this directory. Luckily. Imagine what  
happened if more desktop environments or applications would ship files here.
Imagine what happened if e.g. KDE would use xmodmap too and put its own
files here, most likely with conflicting file names and different contents.

While themes, icon looks and plenty of other things can be specific to one
desktop environment or one setting, there are things that must be unique to
all of them. As probably no-one wants to duplicate the timezone or iconv and
localedata database found in glibc, instead every application uses the
interface provided by glibc, similarly keyboard layouts mustn't be specific
to one desktop environment or another. It is a completely brain-damaged idea
that gnome-applets ships a set of keyboard layouts, and then why doesn't KDE
or icewm or wmaker do so? And what if they all shipped their layouts?
Obviously they would all be different, causing a lot of unnecessary troubles
to the user. Keyboard layouts mustn't be specific to the taste of Gnome
developers, they should be the same as the keyboard layouts in every desktop
environment. And the only possible place to store these official layouts is
obviously XFree86.

Take a look at the keyboard definitions found in XFree and in Gnome. You'll
find that xfree's version is much better, much up to date and has more
features. For example see the Hungarian layouts. XFree defines 16 variants
(2^4), 8 of them are obsolete latin1 mapping, but 8 are fairly usable, and 
their content is trivial based on their name (e.g. uni_101_qwertz_comma is
for 101-key keyboards, qwertz row, and comma on the numpad). gkb offers     
approx. 3 obsolete and 3 usable Hungarian layouts, where it isn't clear at  
all which one I should choose and how it will behave. XFree's layouts
already include the Euro symbol, while gkb's layouts do not.

So I hope that by now you see that XFree has well maintained layout files   
and it's completely impossible for Gnome to stay up in sync with it, and
that it is not the good approach. Layouts should not be maintained at all
within Gnome, only in XFree, and Gnome should use those ones. It is only
possible with setxkbmap, not with xmodmap.

And there is one more, completely different argument against xmodmap. When 
using xmodmap, you have to know the physical keycodes. These are hardcoded
to gkb's xmodmap files, and hence they only work on PC's. Different
computers (e.g. a Sun, IBM RS* etc.) have totally different physical
keycodes and hence gkb cannot be used at all on these computers, as it loads
a nonsense layout. If you think of X as a networking system where different 
machines can play the role of the server and the client (e.g. the X server  
where I'm sitting can be a Sun while the X client where Gnome runs can be a
PC), you see that it is impossible for the client (e.g. xmodmap or gkb or
gnome) to know what physical keycodes the keyboard produce for each key.
And hence it is impossible for them to set a correct layout using xmodmap.  
It is only the X server that knows the type of the keyboard, and so the X
server has to be asked to load a particular keymap. And this is exactly what
setxmbmap is for. Xmodmap is unable to complete this job because of its 
purely client side design.