In really simple terms and a gross generalization...
U can start an Intel chipset at the enthusiast top with Xxx, X58 for example. 58 being the key. X being...... pfft eXtreme ??
P .... Performance ? Gah. There are probably official terms for them.
U'll then eventually get:
58, 55, 53, 51 - each with a feature taken away from the former. In some cases something added in it's place. U could lose some slot abilities on a 55 over a 58, but gain on perhaps a revision of the 5 series, lessons learnt since the 58 was rushed out (or something not finished in time, a memory or bus/bios tweak). In some series, it has been the other way around.
By the time the enthusiasts have snapped up the 55(s), Intel mostly look forward to 6 series development, but will continue to produce lesser, 53, 51, with sometimes only minor abilities removed. Sometimes the 53 can turn out to be an absolute bargain of a clocking mobo. Not expected to be a performer by the maker, it'll usually have little or no onboard cooling.
The G often dictates some kind of onboard graphic, although I have seen some manufacturers create a mobo without. I would only assume here that whilst Intel intended the release to be for onboard graphic mobos, the manufacture saw the chance to grab the cheaper chip and make one without graphics anyway. Costing and performance dependent I would expect.