Is there a GUI design app for the Tkinter / grid geometry?
You have VisualTkinter also known as Visual Python. Development seems not active. You have sourceforge and googlecode sites. Web site is here.
On the other hand, you have PAGE that seems active and works in python 2.7 and py3k
As you indicate on your comment, none of these use the grid geometry. As far as I can say the only GUI builder doing that could probably be Komodo Pro GUI Builder which was discontinued and made open source in ca. 2007. The code was located in the SpecTcl repository.
It seems to install fine on win7 although has not used it yet. This is an screenshot from my PC:
By the way, Rapyd Tk also had plans to implement grid geometry as in its documentation says it is not ready 'yet'. Unfortunately it seems 'nearly' abandoned.
Apart from the options already given in other answers, there's a current more active, recent and open-source project called pygubu.
This is the first description by the author taken from the github repository:
Pygubu is a RAD tool to enable quick & easy development of user interfaces for the python tkinter module.
The user interfaces designed are saved as XML, and by using the pygubu builder these can be loaded by applications dynamically as needed. Pygubu is inspired by Glade.
Pygubu hello world program is an introductory video explaining how to create a first project using Pygubu.
The following in an image of interface of the last version of pygubu designer on a OS X Yosemite 10.10.2:
I would definitely give it a try, and contribute to its development.