Clearly, here TUI solution and GUI solution are completely different. I'd keep things simple, instead to try to merge them. Other scripts use if/else to handle multi frontend. I would add something like:

if [ $DIALOG == "gbsddialog" ]
	timezone_gbsddialog
else
	tzsetup
fi

This should avoid to change finalconfig.

Some script uses $USE_XDIALOG, I would avoid this variable because it is related to bsdconfig to handle xdialog (another topic to discuss in the future); $DIALOG == "gbsddialog" is obvious and clear for future developments.

(I' ll add a new case for CLI in the future, something like if [ $DIALOG == "clidialog" ] tzsetup --cli)

This file can be renamed (maybe timezone_gbsddialog), so we can "isolate" GUI code; simple and clear for future changes/developments/improvements.

Here I would add a comment for rationale and future developments, maybe:

#Copy and adapt 'bsdconfig timezone' for gbsddialog because bsdconfig is
#designed for xdialog. ...etc...
#In the future we can add GUI lib to tzsetup ...etc....
#What you changed in this file ...etc...
#Rationale ...etc...

I would want to understand the gbsddialog autosizing and position.

Does gbsddialog provide autosizing? Does it need bsdconfig sizing API?
Is gbsddialog suitable for bsdconfig (that is xdialog) autosizing? For example xdialog has buttons below while gbsddialog in the title bar, how is this handled?

Does gbsddialog need bsdconfig auto position API? Does it provide auto position?

In this file gbsddialog instead of $DIALOG can be sufficient.
(As guideline for bsdinstall we can avoid to use var $DIALOG when we want to use only one frontend.)

The TUI code can be deleted, we could avoid reinventing the wheel (tzsetup). Obviously all the code unrelated to gbsddialog can be deleted.

Actually here my intention is not to separate the code between the TUI and GUI solutions. I am simply decoupling setting the date & time step from the one setting the timezone, for clarity. It happened to me once that I wanted to set the date & time in the finalconfig step, while knowing that the timezone was correct, and therefore I thought this was no longer possible.
Therefore there should be no need to detect GUI vs TUI in this patch.

I have implemented the auto-sizing of gbsddialog so that it works exactly like in bsdconfig. The algorithm is currently as follows:

• obtain the current workarea from Gtk+: src/common.c, get_workarea()
• obtain the default font size from Pango: src/common.c, get_font_size()
• divide the height twice by the font size, remove 9 lines for the title and bottom
• divide the width by the font size, remove 4 columns for a bit of margin on both sides

This can be seen in src/gbsddialog.c, where the handling of PRINT_MAXSIZE therefore returns a value in columns and rows available with the default font.
In my tests I am usually between 80+ and ~150 columns, and over 25 lines available, which should more than match the 80x25 of a VGA screen.

Regarding positioning, gbsddialog hints the window manager to position the window in the center of the primary screen, as per Gtk+' API gtk_window_set_position(GTK_WIN_POS_CENTER), see _builder_dialog() in src/builders.c.

My aim is to behave completely identically to bsddialog.

I would rather follow @jrtc27's advice and find a way to re-use the timezone code from bsdconfig, instead of keeping a slightly modified copy in bsdinstall as per the current version of this patch. I will look into that.
bsdinstall already depends heavily on bsdconfig, and there should be no perceptible performance impact in comparison with tzsetup.