The tune is either a known tune or a known fictional one. From the entry for 24 August 18114 in the collected memoirs of SIr Walter Scott, vol IV, p 199:

"The chief returns ashore, with his piper playing the Macleod's Gathering, heard to advantage along the calm and placid loch, and dying as it retreated from us."

https://archive.org/details/memoirsoflifeofs04lock_0/page/198