Well GH is only a 4-minute walk from PL on Park Lane & is facing Hyde Park as well. Jan, if you can get GH go for that over PL. I've stayed at both & GH is nicer in terms of property, rooms, and lounge. Better chance of upgrade & has (currently) free booze in the exec lounge (PL is paid). PL is closer to the Marble Arch tube stop, but to me that's about its only advantage, and again it's only a 5-minute walk to GH from there (or for that matter GS). Anyway, GH, GS, & PL are all close to Hyde Park.
Cheers.