The ones starting with U Is the weight of the entire racquet, 2U is the heaviest 3U is the average weight & 4U is the lightest. Grip size can be different for all brands I'm going to use yonex for an example. G3 grip size is big size grip and is mostly sold in the UK I believe so if u have big hand G3 is your friend if you like it. G4 is average lots in Canadian sportcheks here, G5 is smaller then G4 I prefer it even though G4 fits my hand due to more control
Note that there might be differences from the companies, i.e. a G3 grip dont need to have the same thickness for all companies. For instance with Victor rackets before 2013 they used a small number for a thin grip and a big number for a thick grip, so it was reversed in comparison with yonex. But since 2014 the rate it the same way as yonex. The weight 2u, 3u, 4u etc. should be quite similiar with all companies.