It sounds a little bit like working backwards.
Normally I work on an animation in sequence so I get access to onion skinning and I can constantly tap hotkeys to jump back and forth between frames to check volume. Only after I'm happy with the animation do I turn it into a sheet.
Working from the beginning within a sheet seems illogical. The sheet is the end product, the animation is what you are working on, so it makes sense to have your workflow built around the animation, not the sheet.
So yes, I highly recommend GraphicsGale. The free edition will still preview animations, so you can work on them and then make a sheet out of them. You only need to pay for the ability to actually export animated gifs.
But if you're hell-bent on working from sheets and then previewing the animations from the sheet... no program comes to mind for me. It probably wouldn't be too hard to cook up a little script to do the job, though.