G is approved supplier manufactured, comes in LR package and is distributed by LR.
OEM is approved supplier manufactured, comes in OEM packaging and is OEM manufacturer distributed. Not all products are available in OEM form, e.g. proprietary LR designs such as the cam cover gasket.
P is everything else stretching from non-approved but well known manufacturers (although not necessarily building the parts as per LR specs) to obscure chinese outfits. Most of Britpart stuff falls in this category. Quality in this category varies enormously.
Rgds
Matt