After about ten minutes (thank you University of Glasgow archives) I’ve identified the ‘Gibson’ in CKSM’s war letters from 1st Ypres as Captain Robert Gibson. He was apparently a Captain in the 3rd, but attached to the 2nd Battalion of the KIng’s Own Scottish Borderers. The University of Glasgow, where Gibson studied Law and Chemistry, before winning an exhibition to Balliol College, Oxford, has posted this photograph and a short biography at http://www.universitystory.gla.ac.uk/ww1-biography/?id=219. This gives his birth date as 1891, but the CWGC gives his age at death as 26, making him born in 1888 or early 1889.
Gibson was killed during the German assault on Hill 60 on 5 May 1915. It seems likely that he is the unnamed comrade in CKSM’s poem ‘Domum’, written at St Eloi a month later:
What was it you fought for, whose profit that you died?
Here is Ypres burning and twenty towns beside,
Where is the gain in all our pain when he we loved but now
Is lying still on Sixty Hill, a bullet through his brow?
“He died one thing regarding that is better worth
Than the golden cities of all the kings on earth.
Were right and wrong to choose among, he had seen the right,
Had found the thing appointed and done it with his might.”