The best tool for doing layouts using grid, IMHO, is graph paper and a pencil. I know you're asking for some type of program, but it really does work. I've been doing Tk programming for a couple of decades so layout comes quite easily for me, yet I still break out graph paper when I have a complex GUI.
Another thing to think about is this: The real power of Tkinter geometry managers comes from using them together*. If you set out to use only grid, or only pack, you're doing it wrong. Instead, design your GUI on paper first, then look for patterns that are best solved by one or the other. Pack is the right choice for certain types of layouts, and grid is the right choice for others. For a very small set of problems, place is the right choice. Don't limit your thinking to using only one of the geometry managers.
* The only caveat to using both geometry managers is that you should only use one per container (a container can be any widget, but typically it will be a frame).