I've shot various PG with a 300Wby (although not Gemsbok, which I shot with a .375H&H) and was not very impressed with the cartridge. Which is why I later sold it.
Of the various Gemsbok that I have seen in the true wilderness of Kaokoveld and Damaraland in Namibia, quite a few would have been a least easier to shot with a flatter shooting cartridge than say a 30-06 (or 375H&H). This was in open areas with little or no cover. And far from any population save a few Himba families here and there. But, then most Gemsbok hunting in Namibia is done in areas where shots aren't so long, and thus a regular .308 or 30-06 or simular should be fine IMO. In fact, my old mother shot a Gemsbok with her .308 with no problems. Shot placement is what counts!

Erik