I need to do a complicated GAP animation. It involves many steps that I will probably need to redo several times. A script that I could edit and re-run multiple times so that I can get it right.

I don't believe such functionality currently exists unless you write a dedicated plug-in. You could, however, use the 'filter-macro' to apply a set of multiple filters in one pass as long as you do not have to repeat the same filter with different values ('filter-macro' uses the Last Values with which the filter was run).

For example, you could set up a filter macro which in one pass would...

Apply a Gaussian blur with radius of 10 Apply a color curve
Perform a threshold
Perform a gradient map

... upon all of the layers specified in the Frames Modify call.

However you could NOT, within that set, have a second Gaussian blur with a radius of 20 (you could repeat the radius=10 blur though) and you could not include operations which are not filters (such as resize or duplicate the layer).

Overall, it is of very limited utility but, if the situation is just right, it can provide some benefit.