I checked the South Africa Medal Roll for 5th Company Royal Engineers. The five men that Forsyth listed as Gunners are shown under the heading of Sapper. J. Hooper 10528 has the notation "unpaid L. Corporal" above the regimental number. The other four have similar notations. I think Forsyth misread the notation.
I can copy the pages from the medal roll for you, if you like, and email them to you.