Gnome bug #412493 has been there for more than a year, it refers to the ability of handling forms with zenity. There are some proposals, but far too way too complex.
And sometimes, simple problems deserves simple solutions...
As usual, create your form in Glade, we will use the same example described in the bug report, a simple login window for a downloader pretending being connected to ftp.bli.org.
This is the running application.
The entry fields are annotated with autoglade as:
We use this simple script (login.sh) to call autoglade using login.glade