A GLS approach in automatics can only be flown with the approach mode currently and the approach mode (same as VORLOC) is only able to track straight in approaches, not curved ones. Which explains why both RNP (AR) and curved GLS approaches are not possible in approach mode. Of course one could fly a curved approach manually following the normal needles. RNP (AR) can be flown in LNAV/VNAV which can follow curved approach paths, but isn't a precision approach mode.