I would migrate only questions that have the google-spreadsheet tag or can be edited to have the google-spreadsheet tag.
Questions, in combination with the google-apps-script tag are a little bit tricky. The Google Apps Script Developers chose Stack Overflow as their primary Q&A site for related questions. Therefore I would leave them.
Not everybody, on the mod. team of Stack Overflow, thinks those questions belong somewhere else. To give you an example, this flag was declined:
Perhaps we can create a community post over here, with all the pro's and con's, and migrate that to MSO and see what the opinions are?
Use the "other" option when flagging for moderator attention and ask for it to be migrated. However, always bear in mind the first rule of migrations:
Don't migrate crap
So it better be an otherwise excellent question that just happens to be on the wrong site.
However, be aware that there are over 1,500 flags outstanding on Stack Overflow at the moment so it might take a while for yours to be spotted and processed.
Another alternative is to ping me in chat (I'm usually around, but not always "in" the Web Apps chat room) and I'll take a look.
However, the question you linked in your question is older than 60 days and therefore it can't be migrated by anyone. In this case just vote/flag to close as off topic on Stack Overflow using the custom reason to direct the user here.