The answer to most of it is yes. GTK is just a GUI Toolkit used to create the windows, the underlying logical code is written in PHP. Thus, within reason, almost anything you write for Win32 in php-gtk will run on *Nix as well. Some of the GTK functionality hasn't been ported to Win32 yet, but that's not really a big deal as long as you're careful. There is no GTK port for Mac though.... thus PHP-GTK doesn't run on Mac.
You can have the remote client-side apps open a socket or ftp connection to your site and grab source file updates when you've modified the program. However there's only one proggie thus far that will create a Win32 exe file from a PHP script, and development on it is currently stagnated. It doesn't work with PHP-GTK versions higher than 0.0.4. It also can't buffer more than 5,999 chars of code. You must therefore distribute most of your program's source in text files. Unless you purchase and use the Zend Encoder, it will be plain-text and your customers will be able to read/modify the program's logic (possible security hazard). You will also have to distribute a copy of the PHP interpreter and it's supporting libraries, the php-gtk extension, and the GTK runtime libraries.
I'm not sure what you mean when you ask about hardware resources. GTK is only a GUI toolkit... the underlying code is written in whatever language you choose... PHP in this case. GTK does have access to some hardware directly, but this is usually carried out by either GDK or GLib and you have no direct access to those libraries (GTK accesses them for you).
Any further questions you have regarding php-gtk should probably be directed to the one of php-gtk mailing lists (php-gtk-general is the best for this kind of question). I don't think many folks who read PHPBuilder are developing using php-gtk yet, so you're more likely to find a more knowledgable audience on the lists.
-geoff