Kaya
3rd NSWMR (sixth contingent) was in the field between April 1901 and April 1902. During that time, it was attached to Col. Rimington's Column and conducted operations in Eastern Transvaal and Eastern Orange River Colony. Specifically, it took part in drives; especially in the Harrismith area where it captured 231 Boers, and took 26,000 head of cattle and 2,000 horses. That information from Murray's "Official Records".
It is not surprising that you are unable to locate specific information on "G" Sqn. From a reading of Stirling's "The Colonials in South Africa", 3NSWMR appears to have been fielded as an entire Regiment and you will not find the details you want unless you are fortunate to discover a squadron diary of something similar.
To try and trace the spoor of 3NSWMR, perhaps the entries for that regiment in the SAFF casualty list might help. You are also encouraged to read 3NSWMR's entry in Stirling's "Colonials in South Africa".
Otherwise, all I can say about your 2987 Tpr. Charles John Walkling of 3NSWMR is that he did not feature in the casualty lists, hailed from Uralla, NSW and was present at a Welcome Back function held at Armidale, NSW on 20/6/1902.
Regards
IL.