Property of the script line: DiaryGridLines.
To edit this property, in the properties inspector, click on the ellipsis button at the right of the GridTemplate property editor. The templates explorer then comes up and you can select a Template and click OK.
Note that when you scan a GridTemplate, a copy of that file is saved in the Templates database. This means that you can still use a Template, even if the GridTemplate it uses has been moved, deleted or modified. Q++Studio will use the GridTemplate as it was at the time it was last scanned, but will issue a warning in the messages window.