How can I export gmail message subject lines from a search?

You can use MsgExtract to export only the subject lines or any other email field to excel or to a text file.

After you label the search in gmail, you only need to select the same label in MailExporter and then select the email parts that you want the export:

You can find further information about MsgExtract exporting features here:

http://www.maildev.com/msgextract/

(Disclaimer, I am the author of Msgextract)