Had a look to see if I could see any trace of the Long Barrow when visiting the Grickstone Standing Stone. Unfortunately I couldn't see any trace. Looks like it has completely disappeared?
The 'Grickstone' itself is apparently a stone alone with no hint of a barrow. Grinsell and O'Neil'*s source said it "was put up when the Greek wars were in England" and "a Greek officer is supposed to have been buried under it."
Not far away there was Grickstone Farm long barrow at ST782832. Its three chambers were excavated in 1844, revealing many skeletons. You could still see some slabs of stone at its SE end in 1960. The photo on the Megalithic Portal just has it as a bump in the ground. Their photo of the stone though makes that look worth visiting - and it's conveniently next to a footpath.
(*'Gloucestershire Barrows' - Trans Brist Gloc Arch Soc 1960.)