You need to put these in your scripts folder and then reload aseprite to see them, to get at your scripts folder go to File > Scripts > Open Scripts Folder
To use the gradient script make a selection on a none empty layer and then run it from File > Scripts > Gradients
Centers in gradients don't actually do anything yet, center is automatically set to the center of your selections bounding box.
Presets script can be run at any time from File > Scripts > Presets, two big notes, currently the palette will just be black, and there is an issue with the settings file getting corrupted but I haven't been able to properly reproduce these conditions yet.