I've been doing a deep dive of late into Gordon Bok's work and discography, which led me to this song on his 1965 first album 'Gordon Bok' on Verve Folkways. It's the only one of his albums not available in digital form. I was able to obtain a copy of it on eBay in perfect condition. The song is the fifth track on the first side and is listed simply as 'Rhum'.

Of the twelve tracks on that album ten are on his 2015 retrospective 'Then and Now' album, but 'Rhum' isn't one of them, and it hasn't been reprised on any of his other albums, so it's only found on 'Gordon Bok'. Here are the lyrics as I hear them:

Far away seaward thy green land, my youth land,
Far away seaward the Cuillin of Rhum,
Though far in the dreamtide I'm hearin’ hill waters
And laughter o’ streams by the Cuillin of Rhum.

When shall I see thy bold shores in the sunlight,
The heather of hills in the risin’ of morn,
The blowin’ grey sea wind goes east in the morning
To run the wild hills of the Cuillin of Rhum.

I did find one video of it on YouTube:
https://youtu.be/SyoiU210BL8?si=TEfVUZk81zQED-R_
The rest of the tracks on that album are there as well.

Relating Gordon's lyrics to those mentioned above, his first verse is very similar to the Tannahill Weavers' third verse, and his second verse is very similar to their first. Archie Fisher's version closely parallels the Tannahill Weavers'. As for Kenneth MacLeod's version in 'Road to the Isles', Gordon's first verse is similar to its first and third verses. The only parallels to Gordon's second verse are in the posts of Barbara and Julia above who are recalling lyrics as learned from him, so it could well be that he wrote the second verse as Julia states.

There are no liner notes or lyrics with the 'Gordon Bok' album, and the 'About the Songs' notes on the back cover make no mention of 'Rhum' at all. Also there is no mention of the song on Gordon's website, and it is not included in either of his two books of lyrics, 'Time and the Flying Snow' and 'One to Sing, One to Haul'. So there's the lyrics to be heard on the album track but nothing else about it from Gordon that I can find. Maybe he's discussed it in interviews or concerts.

Here is some info on the island of Rùm and the Rùm Cuillin from Wikipedia's excellent page on it: https://en.wikipedia.org/wiki/R%C3%B9m
Rùm (Scottish Gaelic pronunciation: [r?u?m]), a Scottish Gaelic name often anglicised to Rum (/r?m/), is one of the Small Isles of the Inner Hebrides, off the west coast of Scotland, in the district of Lochaber. It is the largest of the Small Isles, and the 15th largest Scottish island. The main range of hills on Rùm are the Cuillin, usually referred to as the Rùm Cuillin, in order to distinguish them from the Cuillin of Skye. For much of the 20th century the name became Rhum, a spelling invented by the former owner, Sir George Bullough, because he did not relish the idea of having the title "Laird of Rum".