Here's a simple example...
width-request="300" is needed to make the column wide so space
will show.
#!/bin/sh
export MAIN='
<window resizable="true">
<hbox width-request="300" homogeneous="true"> <hbox>
<button cancel></button>
<button>
<label>"Set"</label>
<input file stock="gtk-ok"></input>
<action>EXIT:exit</action>
</button>
</hbox> </hbox>
</window>
'
gtkdialog -c -p MAIN