Illustrator gradient help

Louise,

If the filled path was created as a stroked path with variable width, you can just go back to the original and apply the gradient along the stroke (always keep (a copy of) the original artwork when you start destroying it).

If you only have the path as filled and closed, in order to easily and accurately create a middle path that fits the shape, you can use a 1 step blend (can be used for varying width) as follows (Smart Guides are your friends):

1) Object>Blend>Blend Options, set Spacing to Specified Step with 1 step (that will prepare the right blend);

2) Copy the closed path and save the original path elsewhere or lock/hide it (always keep a copy of artwork you are going to destroy);

3) Select everything and switch from Fill to Stroke in the Toolbox (that will give you stroked paths (and you can see what you are doing));

4) First deselect by Clicking an empty spot and then with the Pen Tool Click one end Anchor Point (it will be the path on top), in order to reverse its direction, you may have to redo at the other end, see below;

6) Select everything and Object>Blend>Make;

If no middle path appears:

7) Ctrl/Cmd+Z to Undo 6), then redo 5) clicking at the other end, then redo 6), now the middle path should appear;

If the middle path fails to fit midway between the two parts:

8) Ctrl/Cmd+Z to Undo 6), then add one or more Anchor Points on either or both paths to get the best possible fit, then redo 6):

9) Object>Blend>Expand, then Ctrl/Cmd+Shift+G to Ungroup, then hide or delete both the original paths (that will give you the stroked path midway between the paths), then set the Stroke Weight as desired (you may use variable width).

You can see an older 1 step blend case here with screenshots (see especially post #7):

https://forums.adobe.com/message/8032700#8032700