ExtendScript does not allow you to control the length, angle, or other parameters of gradients. These are all read-only attributes. Script authors can only work with color stops.

For example, to rotate a gradient, we have to use the rotate() method, which has another problem. The angle of such a gradient is not copied to another object with the Eyedropper Tool: https://medium.com/@aiscripts/gradient-rotation-bug-24d7a6f6038f

The fillColor assignment commands (selection[1].fillColor = selection[0].fillColor) carry over the base gradient, losing other properties. This reduces the ability to automate gradients via ExtendScript.

Also ignores other Uservoices:
1) https://illustrator.uservoice.com/forums/908050-illustrator-desktop-sdk-scripting-issues/suggestions/44461230-adjusting-one-gradient-causes-changes-to-unrelated
2) https://illustrator.uservoice.com/forums/908050-illustrator-desktop-sdk-scripting-issues/suggestions/40296880-erroneous-fillcolor-type-one-shapes-with-freeform
3) https://illustrator.uservoice.com/forums/333657-illustrator-desktop-feature-requests/suggestions/38435974-gradient-swatch-includes-angle-and-position-pleas